started user keys support

As a starting point from #340, implemented these two endpoints:

- [x] http://developer.github.com/v3/users/keys/#list-public-keys-for-a-user
- [x] http://developer.github.com/v3/users/keys/#list-your-public-keys
`ApiUrls` has become a big class with everything intermixed. Moving forward, I think we should break it out so that things are easier to navigate.