Feature/oauth2

Adds OAuth2 strategy. This is abstract, in there's no built in Oauth providers leveraging this, but see https://github.com/michaelkirk/discourse_oauth2_example for an example of how you might integrate your existing OAuth2 provider's authentication using a Discourse plugin.

FYI: My use case for this was to use Discourse as a private forum, accessible only to users of an existing Rails app. Read more here: https://github.com/michaelkirk/discourse_oauth2_example