Added git tags support.

[sww]

Added API calls for http://developer.github.com/v3/git/tags/.

[willnorris]

Maybe it's my background in Java, but having a type simply named "Object" makes me nervous :) I'm not sure if we'll see conflicts later on, but how about calling it TagObject?

[willnorris]

nevermind... ignore this. I see that "object" is actually used in several places throughout the API, and fortunately they're all consistent. So simply calling this Object should be fine.

[sww]

Yea I don't feel great about it being Object. What do you think about GitObject?

[willnorris]

yeah, that's probably better.

[sww]

How does that look? It's been a while!

[willnorris]

rebased onto master, cleaned up a little and merged as 4fe63c8

just as FYI, the cleanup I did was to switch struct fields to pointers, and to make it so CreateTag can actually take a Tag as input rather than a TagRequest. It still gets converted into a TagRequest (though that's no longer exported and I renamed it to createTagRequest), but that happens transparently to the caller now. Not sure why I didn't consider doing it that way when I first reviewed this.