Updates and improves functionality

Thanks for authoring this gem! I tried using it today and ran into some issues (tokens weren't working). My first step to debug was to simply remove any method overrides (basically the whole contents of `OmniAuth::Strategies::Harvest` other than the `:client_options` option). Everything magically started working when I removed that, at which point I implemented enough functionality to make this gem OmniAuth schema compliant.

Let me know what you think!