Added Assertion Flow

Here a simple implementation for an Assertion Flow for Doorkeeper.

This lets you define a method to authenticate users coming from 3rd Party applications like facebook, twitter, etc.

__Example:__
```ruby
Doorkeeper.configure do
    resource_owner_from_assertion do
        facebook = URI.parse('https://graph.facebook.com/me?access_token=' + params[:assertion])
        response = Net::HTTP.get_response(facebook)
        user_data = JSON.parse(response.body)
        User.find_by_facebook_id(user_data['id'])
    end
end
```