PERF: Removed the GIL from parts of the TextReader class
The GIL was removed around the tokenizer functions and the conversion function(_string_convert excluded).
## Benchmark:
### Data Generation:

``` python
import pandas as pd
import numpy as np

df = pd.DataFrame(np.random.randn(1000000,10))
df.to_csv('test.csv')
```
### Benchmark Code:

``` python
import pandas as pd
from pandas.util.testing import test_parallel

def f():
    for i in range(4):
        pd.read_csv('test.csv', index_col=0)

@test_parallel(4)
def g():
    pd.read_csv('test.csv', index_col=0)
```
### Before:

```
In [4]: %timeit pd.read_csv('test.csv', index_col=0)
1 loops, best of 3: 2.3 s per loop

In [7]: %timeit f()
1 loops, best of 3: 9.15 s per loop

In [8]: %timeit g()
1 loops, best of 3: 9.25 s per loop
```
### After:

``` pycon
In [6]: %timeit pd.read_csv('test.csv', index_col=0)
1 loops, best of 3: 2.35 s per loop

In [9]: %timeit f()
1 loops, best of 3: 9.55 s per loop

In [10]: %timeit g()
1 loops, best of 3: 4.38 s per loop
```

some windows cythoning errors.

odd they don't show up for you, what platform are you testing on?

```
[pandas3.5] C:\Users\Jeff Reback\pandas3.5>git checkout -b jdeschenes-nogil_csv master
Switched to a new branch 'jdeschenes-nogil_csv'

[pandas3.5] C:\Users\Jeff Reback\pandas3.5>git pull https://github.com/jdeschenes/pandas.git nogil_csv
remote: Counting objects: 7, done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 7 (delta 0), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (7/7), done.
From https://github.com/jdeschenes/pandas
 * branch            nogil_csv  -> FETCH_HEAD
Updating 26db172..2da353e
Fast-forward
 asv_bench/benchmarks/gil.py |  23 ++++
 pandas/parser.pyx           | 269 +++++++++++++++++++++++++++++++-------------
 2 files changed, 213 insertions(+), 79 deletions(-)

[pandas3.5] C:\Users\Jeff Reback\pandas3.5>make
python setup.py build_ext --inplace
make: python: Command not found
Makefile:2: recipe for target `tseries' failed
make: *** [tseries] Error 127

[pandas3.5] C:\Users\Jeff Reback\pandas3.5>python setup.py build_ext --inplace
running build_ext
skipping 'pandas\index.c' Cython extension (up-to-date)
skipping 'pandas\tslib.c' Cython extension (up-to-date)
skipping 'pandas\hashtable.c' Cython extension (up-to-date)
skipping 'pandas\algos.c' Cython extension (up-to-date)
cythoning pandas\parser.pyx to pandas\parser.c

Error compiling Cython file:
------------------------------------------------------------
...
            # in the hash table
            if k != na_hashset.n_buckets:
                na_count[0] += 1
                data[0] = NA
            else:
                data[0] = parser.converter(word, &p_end, parser.decimal, parser.sci,
                                         ^
------------------------------------------------------------

pandas\parser.pyx:1538:42: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...
                        data[0] = NA
            data += 1
    else:
        for i in range(lines):
            COLITER_NEXT(it, word)
            data[0] = parser.converter(word, &p_end, parser.decimal, parser.sci,
                                     ^
------------------------------------------------------------

pandas\parser.pyx:1557:38: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...
                na_count[0] += 1
                data[0] = NA
                data += 1
                continue

            error = to_boolean(word, data)
                             ^
------------------------------------------------------------

pandas\parser.pyx:1680:30: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...
            data += 1
    else:
        for i in range(lines):
            COLITER_NEXT(it, word)

            error = to_boolean(word, data)
                             ^
------------------------------------------------------------

pandas\parser.pyx:1688:30: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...
            if k != false_hashset.n_buckets:
                data[0] = 0
                data += 1
                continue

            error = to_boolean(word, data)
                             ^
------------------------------------------------------------

pandas\parser.pyx:1758:30: Calling gil-requiring function not allowed without gil

Error compiling Cython file:
------------------------------------------------------------
...
            if k != false_hashset.n_buckets:
                data[0] = 0
                data += 1
                continue

            error = to_boolean(word, data)
                             ^
------------------------------------------------------------

pandas\parser.pyx:1778:30: Calling gil-requiring function not allowed without gil
building 'pandas.parser' extension
C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\amd64\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MD -Ipandas/src/klib -Ipandas/src -IC:\Miniconda\envs\pandas3.5\lib\site-packages\numpy\core\in
clude -IC:\Miniconda\envs\pandas3.5\include -IC:\Miniconda\envs\pandas3.5\include "-IC:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE" "-IC:\Program Files (x86)\Microsoft Visual Studio 1
4.0\VC\ATLMFC\INCLUDE" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\NETFXSDK\4.6\include\um" "-IC:\Program Files (x86)\Windows Kits\10\in
clude\10.0.10240.0\shared" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\um" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\winrt" /Tcpandas\parser.c /Fobuild\temp.win
-amd64-3.5\Release\pandas\parser.obj
parser.c
pandas\parser.c(1): fatal error C1189: #error:  Do not use this file, it is the result of a failed Cython compilation.
error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\BIN\\amd64\\cl.exe' failed with exit status 2

[pandas3.5] C:\Users\Jeff Reback\pandas3.5>
```
 I fixed a few issues with the build
 builds ,couple of errors on windows

```
[pandas3.5] C:\Users\Jeff Reback\pandas3.5>nosetests -A "not network" pandas\io\tests\test_parsers.py
...........S.........................................................................F......................................S....................S..........................S...........................
................................F........S..................................................................................................................................S...........................
..............................................
======================================================================
FAIL: test_parse_bools (pandas.io.tests.test_parsers.TestCParserHighMemory)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\Jeff Reback\pandas3.5\pandas\io\tests\test_parsers.py", line 1177, in test_parse_bools
    self.assertEqual(data['A'].dtype, np.bool_)
AssertionError: dtype('O') != <class 'numpy.bool_'>

======================================================================
FAIL: test_parse_bools (pandas.io.tests.test_parsers.TestCParserLowMemory)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\Jeff Reback\pandas3.5\pandas\io\tests\test_parsers.py", line 1177, in test_parse_bools
    self.assertEqual(data['A'].dtype, np.bool_)
AssertionError: dtype('O') != <class 'numpy.bool_'>

----------------------------------------------------------------------
Ran 446 tests in 12.620s

FAILED (SKIP=6, failures=2)
```
 This was an issue with python3 and not limited to windows.
 @jdeschenes 
- couple of questions for you above. 
- can you post the asv results here.
- pls add a note in the performance section (use this PR number)
- squash
 @jreback 

Where do I need to add information in the performance section? Is it in the what's new file in the documentation?
 whatsnew/v0.17.1 (Performance section)
 @jdeschenes looks good. can you
- add a note in whatsnew (performance section)
- rebase
- squash
 can you update according to comments
 What's the status on this @jdeschenes?  I'd like to include this work in a talk happening tomorrow.  It'd be awesome to be able to say that this was in master rather than in a branch.
 you can say slated for 0.17.1 :)
 I will get the final changes tomorrow.
 @jreback, The changes have been implemented. Let me know if there is anything else that needs to be done.
 @jdeschenes thanks, just some small comments. ping when pushed. pls also post a short benchmark in the top of the PR (you can just run before/after in ipython via timeit if you want), mainly for posterity.
 @jreback: Added the benchmarks.
 looks good

ping when green
 Ping
 thanks @jdeschenes 

and @mrocklin for the pings!
