add comparisons operator to the Period object and fixes issue 2781
This commit should fix [issue 2781](https://github.com/pydata/pandas/issues/2781). The Period object couldn't be sorted because it didn't have any **le**, **gt**, **lt**, **ge** or **cmp** methods.

Can you add a test that asserts an exception is raised (`assertRaises`) if two periods with different frequencies are compared?
 I did some changes in the `__eq__` method and updated the tests accordingly. I think that comparing a Period object with anything, other than a Period object, should raises a Type error. Correct me if I'm wrong.
 Thanks. Note for the future you could have less code duplication by creating a function factory and using the built-in `operator` module
 I can redo it with a function factory if you wish. :)
 @moisan this mergable?
@wesm ok?
 I believe this is mergeable.
 Merged. Thank you!
