BUG: Count on resampled PeriodIndex fails
``` python
In [38]: pd.Series(1, index=pd.period_range(start='2000', periods=100)).resample('M').count()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-38-295afa97781f> in <module>()
----> 1 pd.Series(1, index=pd.period_range(start='2000', periods=100)).resample('M').count()

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in f(self, _method)
    473 
    474     def f(self, _method=method):
--> 475         return self._groupby_and_aggregate(None, _method)
    476     f.__doc__ = getattr(GroupBy, method).__doc__
    477     setattr(Resampler, method, f)

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _groupby_and_aggregate(self, grouper, how, *args, **kwargs)
    353 
    354         if grouper is None:
--> 355             self._set_binner()
    356             grouper = self.grouper
    357 

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _set_binner(self)
    202 
    203         if self.binner is None:
--> 204             self.binner, self.grouper = self._get_binner()
    205 
    206     def _get_binner(self):

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _get_binner(self)
    210         """
    211 
--> 212         binner, bins, binlabels = self._get_binner_for_time()
    213         bin_grouper = BinGrouper(bins, binlabels)
    214         return binner, bin_grouper

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _get_binner_for_time(self)
    538         if self.kind == 'period':
    539             return self.groupby._get_time_period_bins(self.ax)
--> 540         return self.groupby._get_time_bins(self.ax)
    541 
    542     def _downsample(self, how, **kwargs):

/Library/Frameworks/Python.framework/Versions/3.5/lib/python3.5/site-packages/pandas/tseries/resample.py in _get_time_bins(self, ax)
    905         if not isinstance(ax, DatetimeIndex):
    906             raise TypeError('axis must be a DatetimeIndex, but got '
--> 907                             'an instance of %r' % type(ax).__name__)
    908 
    909         if len(ax) == 0:

TypeError: axis must be a DatetimeIndex, but got an instance of 'PeriodIndex'
```

Expected:

```
Out[39]: 
2000-01    31
2000-02    29
2000-03    31
2000-04     9
Freq: M, dtype: int64
```

On pandas 0.18
