BUG: correct Period comparisons
closes #7376

@jreback @jseabold comments? passes except a random failure on 2.7
 looks ok
 I'm not sure I really understand the implications of the change, but looks harmless enough?

Should probably go in the release notes if for some reason someone was relying on the TypeError being raised. The rest of the magic methods still raise a TypeError, though I doubt it's worth worrying about consistency here? I can't see much of a reason for it.
 yep there will be a doc blurb in v0.14.1.txt
