OAuth Gadgets - Moved to new Rails

This pull request moves over OAuth gadget providers feature in Go to Rails 4. It was already a Rails engine but not a mountable engine, and was not working. Now, it is still not a mountable engine, but it works! Was waiting on the OAuth clients pull request before merging this, and fixing conflicts.

I've tried it with a Mingle gadget, and tried out the card activity page, in the compare pipeline feature.
As part of oauth2_provider engine I had added these variables in ```environment.rb```. It should be available to your plugin as well.