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
As part of code reviews, if you're modifying/adding URLs - move it out to a separate partial class.