BUG: groupby().size gives casting error on 32 bit platform
```
In [1]: df = pd.DataFrame({"id":[1,2,3,4,5,6], "grade":['a', 'b', 'b', 'a', 'a', 'e']})

In [2]: df.groupby("grade").size()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-d8e387418f9d> in <module>()
----> 1 df.groupby("grade").size()

/home/joris/scipy/pandas/pandas/core/groupby.pyc in size(self)
    818 
    819         """
--> 820         return self.grouper.size()
    821 
    822     sum = _groupby_function('sum', 'add', np.sum)

/home/joris/scipy/pandas/pandas/core/groupby.pyc in size(self)
   1380         """
   1381         ids, _, ngroup = self.group_info
-> 1382         out = np.bincount(ids[ids != -1], minlength=ngroup)
   1383         return Series(out, index=self.result_index)
   1384 

TypeError: Cannot cast array data from dtype('int64') to dtype('int32') according to the rule 'safe'

In [4]: pd.__version__
Out[4]: '0.17.0rc1+108.g3fb802a'
```
