Convert to use Faraday

This has several reasons. For one: Faraday is nicer for library users in that we can give access to the faraday object so that they can configure which adapter to use, in case they want to do tricky things like parallelism.

Secondly, Faraday is just a much nicer API to use than the OAuth gem from the perspective of a gem author and maintainer. And, relatedly, Faraday covers more of the HTTP feature set, so we should be able to make requests however we see fit, rather than using hacks as we do in a couple of places in the current release. And, since there's an existing OAuth middleware for Faraday, we shouldn't have to require library users to jump through any more hoops.