Custom faraday and verify method

I've somehow managed to refresh this page twice and lose the long description of what this is doing, so going for a tl;dr now...

*Benefits*
1) Allows us more freedom with `faraday`, without having to make changes upstream on `faraday` itself or on `digital_ocean`
2) Fixes #53, now uses the Faraday ClientError wrapper so you can return a clean response instead of a `nil class` stack trace (as seen in #52)
3) Adds a new verify option, just because it was a nice way of adding a test for my changes and its useful to check your credentials haven't been revoked without running authorize again.

@blom @pearkes any thoughts?