ENH: add expression evaluation functionality via eval
closes #3393.

cc @jreback
# `eval` Roadmap
## PyTables

allows natural syntax for queries, with in-line variables allowed

some examples
`"ts>=Timestamp('2012-02-01') & users=['a','b','c']"`

`['A > 0']`

```
dates=pd.date_range('20130101',periods=5)
'index>dates'
```

Todos:
- [ ] update docs / need new examples
- [ ] API changes (disallow dict, show deprecations for separated expressions)
- [ ] tests for `|` and `~` operators, and invalid filter expressions
- [ ] code clean up, maybe create new `Expr` base class to opt in/out of allowed operations
    (e.g. `Numexpr` doesn't want to allow certain operations, but `PyTables` needs them,
     so maybe define in the base class for simplicity, and just not allow them (checked in `visit`),
     which right now doesn't allow a `generic_visitor`)
- [ ] can `ops.Value` be equiv of `ops.Constant`?
## Documentation
- [ ] enhancing performance section
- [x] `eval` docstring
- [ ] Warn users that this is _not_ a magic bullet
  - E.g., `x = 3; y = 2; pd.eval('x // y', engine='python')` is 1000 times slower than the same operation in actual Python.
## Engines
- [x] python engine for testing (basically a restricted eval, should make sure nothing dangerous a la `eval('os.system("sudo rm -rf /")')`) is possible here
- [x] numexpr engine

Should python be the default engine for small frames?
## Functions
- [x] toplevel `isexpr`
- [x] move `eval` to top-level pandas
## Error Handling
- [x] syntax error handling
- [x] name error handling
## Alignment
- Algorithm
  1. flatten the tree of terms
  2. resolve the variables, special casing only numpy arrays/scalars/single unary operations (probably room for more  abstraction here)
  3. align by joining on the indices using the current broadcasting behavior
  4. update the original namespace with the newly aligned version of the underlying numpy array
  5. reconstruct the final object with the aforementioned joined indices and type
- [ ] punt to `'python'` engine when a `Series` and `DataFrame` both have `DatetimeIndexes` (I would rather not do this and just enforce the to-be-deprecated index-aligning-if-both-indices-are-datetimeindex behavior)
- [ ] `PeriodIndex`es don't work (well, they won't pass my tests) because of a stack overflow bug when joining a `DatetimeIndex` and a `PeriodIndex` (that bug is slated for 0.13, but a design decision needs to be made)
- [ ] alignment for `NDFrame` (is this really necessary?)
- [x] unary alignment
- [x] binary alignment
## Scope
- [x] deal with global scoping issues (not sure if this is tested well enough...which means probably not)
- [x] allow `Expr` objects to be passed to `eval` (`Expr` will pull in the local and global variables from the calling stack frame)
## Miscellaneous
- [ ] replace current `evaluate` function
- [ ] `Expr` in `NDFrame.__getitem__` cc @jreback
- [x] reconstruct the final object to return the expected output type, e.g., `df + df2` should return a `DataFrame` if `df` and `df2` are both `DataFrame` objects.
- [x] change `Expression` to `Expr`
- [x] add `truediv` keyword to `Expr`, default to `True`
## Operators
- [x] `%`
- [x] `**`
- [x] unary ops don't play well when they are operands of binary ops
## Tests
- [ ] `DatetimeIndex` and `PeriodIndex` objects are untested because of a few join and alignment issues.
- [ ] rewrite a few tests in the python evaluator to make sure the `//` operator works as it should (it works, but there may be edge cases that I haven't though of yet)
- [ ] `//` `numexpr` only supports scalar evaluation here: variables are not allowed so this will always eval in python space (half-assed it a bit in the tests here need to fix that)
- [x] truediv keyword tests
- [x] add tests for nans
- [x] fix `**` associativity issue in testing
## Near Future
- allow `and`, `or` and `not` aliases for the operators `&`, `|` and `~`, respectively
- attribute accessing e.g., `df.A` syntax
- reductions (`sum` and `prod`)
- math ops (sin, cos, tan, sqrt, etc.)
## Far Future
- non-python indexing (or just punt to python)

excellent. can i rebase and push ... not sure if that's going to cause a bunch of merge conflicts when you pull down...?
 go ahead and try.....
 looks ok to me
 actually it interleaved our commits (which is actually correct)
 u could pull down without any pain?
 yeah i know that was nice
 and travis working on it.....and i think you rebased off of current master...as using thew travis scheme :)
 i did `git rebase --interactive upstream/master`; i do that so i don't have to remember where the merge parent is by SHA1
 well actually `grbi upstream/master` cuz who wants to type out that thing all the time?!
 [this](https://www.kernel.org/pub/software/scm/git/docs/git-rebase.html#_recovering_from_upstream_rebase) (hard case) will be needed i think....sorry. i guess there's really no way to recover gracefully if you've squashed or fixed up things. won't do anymore....maybe we should just wait until we're finished to do that...
 I think we can rebase our own commits without too much problems?

not a big deal to rebase at the end (or as long as we coordinate)
 hmm.....actuall my history is now different that the main branch, I don't see the reorderd commits.....?? (mine are in the original order)
 did u get a big merge commit when you pulled? that's the issue...i rewrote history so i think then git finds the nearest parent and tries to merge since it can't know that i deleted N commits in between ... probably least confusing to just wait until the end to squash
 hm you're right i see the same thing my original order

```
* e039ce5 - (HEAD, upstream/eval-3393, eval-3393) COMPAT: allow prior 0.12 query syntax for terms, e.g. Term('index','>',5) (and show deprecation warning) (17 minutes ago) <jreback>
* 1029dc1 - BUG: py3 fixes; revise scoping rules to be more broad      record variable states from the furthermost part of the stack      to the most recent, overwriting if shadow variables occur (17 minutes ago) <jreback>
* 2f87264 - TST: fixed remaining tests (17 minutes ago) <jreback>
* 4abe3c5 - BUG: fixed scoping issues by _ensure_term at the top-level (17 minutes ago) <jreback>
* bb7d620 - ENH: use non_implemented function call in ExprVisitor (17 minutes ago) <jreback>
* 2faeeac - BUG: process visit_Index (17 minutes ago) <jreback>
* de28b79 - BUG: added HDFStore to inherit from Stringmixin (17 minutes ago) <jreback>
* 7830af8 - TST: more test changes (17 minutes ago) <jreback>
* 1f208b4 - WIP: conditions working now, filtering still only ok      good parsing of attributes, subscripting, e.g df.index[0:4] works! (17 minutes ago) <jreback>
* 706276d - WIP: still some debugging statements in (17 minutes ago) <jreback>
* 7ba3aaa - ENH: initial commit for adding Expr based terms for pytables support (17 minutes ago) <jreback>
* e117f1a - ENH: rewrite assignment as equal comparison (17 minutes ago) <Phillip Cloud>
* 16cb733 - ENH: add modest type inference (17 minutes ago) <Phillip Cloud>
* 48719b3 - CLN: generlize operator/expression printing (17 minutes ago) <Phillip Cloud>
* 73415c4 - CLN/TST: fail with a NotImplementedError on Python boolean operators (17 minutes ago) <Phillip Cloud>
```
 http://git-scm.com/book/en/Distributed-Git-Contributing-to-a-Project

I don't think we can rebase at all...just pull/push/merge
 why don't you create a commit and try pushing...then i'll do the same
 it's annoying that the embedded HTML travis tag on github says failing when `master` is passing fine, but this isn't :(

pushed a commit...
 arg it's a stupid network yahoo test failure...
 i pulled down your commit....what am I donig wrong?

```
[sheep-jreback-~/pandas] git push https://github.com/pydata/pandas.git origin/eval-3393
Username: 
Password: 
To https://github.com/pydata/pandas.git
 ! [rejected]        origin/eval-3393 -> origin/eval-3393 (non-fast-forward)
error: failed to push some refs to 'https://github.com/pydata/pandas.git'
To prevent you from losing history, non-fast-forward updates were rejected
Merge the remote changes (e.g. 'git pull') before pushing again.  See the
'Note about fast-forwards' section of 'git push --help' for details.
```
 force push it
 I have something wrong it updated to the `eval4` branch and not `eval-3393`....?
 hm why not just do `git branch --set-upstream-to=origin/eval-3393` and avoid having to type that long command?
 I did that....

git push doesn't do anything then....hmmm
 ok how about `git stash && git checkout -b eval-N --track origin/eval-3393 && git stash pop`...not sure why that should be different than just manually setting the upstream ...
 merged into master? that doesn't seem like a great idea just yet...
 oh nevermind something else is going on
 no....it didn't actually merge
 yeh...that's weird...nothing showed up...hmmm

(I didn't mean to do that)...but I am not sure what actually happened
 in my eval5 branch this is what I did

`git push -u https://github.com/pydata/pandas.git origin:eval-3393 -f`
 it looks like it merged master back into this branch somehow....

see if you can resurrect somehow (maybe create a new branch)..

I don't think master actually changed..
