Implemented the GitHub.IsFollowing() function.
Fixed the request-construction logic for HTTP PUT and DELETE requests,
and renamed the methods to be "http<Method>".

Apparently, you cannot pass "nil" around in place of a pointer to an
object, so I had to clean up the logic and explicitly check to see if
`nil` got passed in as the `content *bytes.Buffer` argument.
Implemented the GitHub.Follow() and GitHub.Unfollow() methods, complete
with tests.