Implement resampling of objects with PeriodIndex
```
In [6]: ts = ts.resample('M', kind='period')

In [7]: ts
Out[7]: 
Jan-1990   -0.008004
Feb-1990   -0.117806
Mar-1990    0.291086
Apr-1990   -0.298024
May-1990   -0.306769
Jun-1990    0.173145
Jul-1990    0.144821
Aug-1990   -0.238503
Sep-1990    0.102926
Oct-1990   -0.023604
Nov-1990    0.043893
Dec-1990   -0.229339
Jan-1991   -0.026023
Feb-1991    0.133059
Mar-1991   -0.006739
Apr-1991    0.138948
May-1991    0.150813
Jun-1991    0.208500
Jul-1991   -0.015446
Aug-1991    0.040145
Sep-1991   -0.252116
Oct-1991    0.378951
Nov-1991    0.333716
Dec-1991   -0.067042
Jan-1992   -0.066540
Feb-1992    0.271326
Mar-1992    0.075388
Apr-1992    0.014510
May-1992    0.008858
Jun-1992   -0.082096
Jul-1992    0.157964
Aug-1992   -0.060613
Sep-1992    0.093209

In [8]: ts.resample('A')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/Users/wesm/code/pandas/<ipython-input-8-d20eb06885e9> in <module>()
----> 1 ts.resample('A')

/Users/wesm/code/pandas/pandas/core/generic.pyc in resample(self, rule, how, axis, as_index, fill_method, closed, label, kind)
    158         idx = self._get_axis(axis)
    159         if not isinstance(idx, DatetimeIndex):
--> 160             raise ValueError("Cannot call resample with non-DatetimeIndex")
    161 
    162         grouper = TimeGrouper(rule, label=label, closed=closed,

ValueError: Cannot call resample with non-DatetimeIndex
```
