ENH: use size instead of cythonized count for fallback cases
- use size instead of cythonized count for integer case since we cannot have
  `nan` in that case
- not faster perf wise
- compilation time is shorter because count has no int templates except for
  dates and times
- any precision issues (don't think there were any) with other integer types are gone, since only `int64` is compared with dates, whereas lower precision integers use `size`.

Anywho, here's the vbench results for this PR vs master:

```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
groupby_int_count                            |   4.0247 |   4.0337 |   0.9978 |
groupby_multi_count                          |   7.5080 |   7.4813 |   1.0036 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
```

@jreback ok to merge?
 actually let me add a test in for lower prec ints
 looks fine...maybe squash and merge
 issues on windows:
- I think the fallback is not working, e.g. should be `x.size`, not `x.size()`; so that is untested
- I think their is a casting issue somewhere here (e.g. regular floats are not getting upcasted to float64), default on windows 32 is float32

```
>>> c:\users\jeff reback\documents\github\pandas\build\lib.win32-2.6\pandas\core\groupby.py(107)f()
-> result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
(Pdb) l
102             try:
103                 return self._cython_agg_general(alias, numeric_only=numeric_only)
104             except AssertionError as e:
105                 raise SpecificationError(str(e))
106             except Exception:
107  ->             result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
108                 if _convert:
109                     result = result.convert_objects()
110                 return result
111
112         f.__doc__ = "Compute %s of group values" % name
(Pdb) p self._cython_agg_general(alias,numeric_only=numeric_only)
*** ValueError: ValueError("Buffer dtype mismatch, expected 'float64_t' but got 'float'",)
(Pdb) d
> c:\users\jeff reback\documents\github\pandas\build\lib.win32-2.6\pandas\core\groupby.py(2405)aggregate()
-> result = self._aggregate_generic(arg, *args, **kwargs)
(Pdb) d
> c:\users\jeff reback\documents\github\pandas\build\lib.win32-2.6\pandas\core\groupby.py(2464)_aggregate_generic()
-> return self._aggregate_item_by_item(func, *args, **kwargs)
(Pdb) d
> c:\users\jeff reback\documents\github\pandas\build\lib.win32-2.6\pandas\core\groupby.py(2508)_aggregate_item_by_item()
-> raise errors
```

```
======================================================================
ERROR: test_mixed_type_join_with_suffix (pandas.tools.tests.test_merge.TestMerge)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tools\tests\test_merge.py", line 686, in test_mixed_type_join_with_suf
fix
    cn = grouped.count()
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 729, in count
    return self._count().astype('int64')
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 107, in f
    result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2405, in aggregate
    result = self._aggregate_generic(arg, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2464, in _aggregate_generic
    return self._aggregate_item_by_item(func, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2508, in _aggregate_item_by_item
    raise errors
TypeError: 'int' object is not callable

======================================================================
ERROR: test_count (pandas.tests.test_groupby.TestGroupBy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tests\test_groupby.py", line 2012, in test_count
    count_as = df.groupby('A').count()
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 729, in count
    return self._count().astype('int64')
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 107, in f
    result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2405, in aggregate
    result = self._aggregate_generic(arg, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2464, in _aggregate_generic
    return self._aggregate_item_by_item(func, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2508, in _aggregate_item_by_item
    raise errors
TypeError: 'int' object is not callable

======================================================================
ERROR: test_count_object (pandas.tests.test_groupby.TestGroupBy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\tests\test_groupby.py", line 2026, in test_count_object
    result = df.groupby('c').a.count()
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 729, in count
    return self._count().astype('int64')
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 107, in f
    result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2063, in aggregate
    result = self._aggregate_named(func_or_funcs, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 2147, in _aggregate_named
    output = func(group, *args, **kwargs)
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 107, in <lambda>
    result = self.aggregate(lambda x: npfunc(x, axis=self.axis))
  File "c:\Users\Jeff Reback\Documents\GitHub\pandas\build\lib.win32-2.6\pandas\core\groupby.py", line 725, in <lambda>
    _count = _groupby_function('_count', 'count', lambda x, axis=0: x.size(),
TypeError: 'int' object is not callable

```
 @jreback small favor: do you have a box or something that i can log into and test this? i don't have a copy of windows anywhere that i can test this stuff on
 wish I had an easy way for you to do that.
 ok i'll see if i can repro this on linux
 the first one I think you can 'fake' a test (e.g. force the cython agg to fail), then the rest raises

maybe passing in float32 should fail the othe test

lmk
 I can test when you have changes
 @jreback does `np.empty(n)` default to `float32` on Win32?
 same on 2.7-64 & numpy 1.8.0

```
C:\Users\Jeff Reback\Documents\GitHub\pandas>c:\python26-32\python.exe
Python 2.6.6 (r266:84297, Aug 24 2010, 18:46:32) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.__version__
'1.6.2'
>>> np.empty((1,1)).dtype
dtype('float64')
>>> np.empty(1).dtype
dtype('float64')
>>> np.arange(1).dtype
dtype('int32')
>>> np.arange(1.).dtype
dtype('float64')
>>>
```
 thx
