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. 
I can change it to `application_scope`? I don't think `allowed_scopes` is quite right either since it's also the default scope that gets assigned to an application if it doesn't have `scope` in its token request.