Fixes doc about /user/keys call.

This library doesn't yet implement the API call that had been referenced: http://developer.github.com/v3/users/keys/#list-public-keys-for-a-user
Merge pull request #222 from patcon/patch-1

Fixes doc about /user/keys call.
Adds user_keys method

http://developer.github.com/v3/users/keys/#list-public-keys-for-a-user