GitHub client_id is required to run Gittip; #398

In marshalling a request to GitHub, we were using os.environ.get, so
that if the GITHUB_CLIENT_{ID,SECRET} keys were missing we'd have
`None`s in the params. I'm not sure what behavior that'd give us (don't
we want str there?), but in any case we want to require those keys to be
present in the environment for Gittip to function. If they're missing
it's a bug.