ENH: support Period.__contains__
supporting ``in`` for a ``Timestamp`` in a ``Period`` is straightforward and useful. Note that we current consider both endpoints inclusive.

```
In [28]: p = Period('2016',freq='M')

In [29]: p
Out[29]: Period('2016-01', 'M')

In [30]: p.start_time
Out[30]: Timestamp('2016-01-01 00:00:00')

In [31]: p.end_time
Out[31]: Timestamp('2016-01-31 23:59:59.999999999')

In [32]: p.start_time in p
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-32-b4a9c4196274> in <module>()
----> 1 p.start_time in p

TypeError: argument of type 'Period' is not iterable
```