Oauth trust levels

This creates three 'trust levels' for OAuth applications based #107. It also allows a `max_scope` to be set for each application, allowing the types of scopes requested by them to be restricted. 

When an application asks for a scope or uses a grant type it is not allowed to use a `400 Bad Request is returned`.

I'm working on adding an admin interface for applications. 
Should [doorkeeper](https://github.com/doorkeeper-gem/doorkeeper/blob/0d0e29e384186d27ef7b1919e59f2114435c0e52/lib/doorkeeper/server.rb#L20-L22) use the controller params instead of the request params? Perhaps a PR?