Add API authentication helper.

This is a small helper service that takes Basic Auth credentials and turns them into custom HTTP headers that an API can look at.

Eventually this will grow to support OAuth credentials instead (but probably not in this PR).
Not entirely but I think we'll have to write our own ``verify_request`` and
validator. I'm hoping that given a bit more time we can contribute
something to oauthlib since most of it seems really nicely split out.