Implemented GetUser

The *GitHub.GetUser() function calls the /user/:user endpoint.
Updated the README with contribution notes
Got back to a good, working state where tests pass
Split the callGithubApi func into three, smaller funcs

callGithubApi() now calls call(), which in turn calls updateRates()
The gothub.call() func is no longer bound to a *GitHub struct
Currently-authed user's emails!

Tests pass, too!
Changed it so that Emails() is a func attached to *GitHub, and not *User

The API only supports getting the currently-authenticated user's email
addresses, so it would seem prudent to not provide is as a method of a
User struct, but rather as a method on the GitHub struct.

Modified tests to match.
Implemented the GitHub.Emails() function.

It currently uses the data from the default response. It doesn't use
the GitHub-specific "vnd" Content-Type.
Changed the mechanism for what's considered a "successful" HTTP response.