Add HTML structure for a welcome message

When using OAuth, you want a welcome message to explain to the user
why they're here.  This implements the welcome message, contingent
on passing a @client variable that contains information about
the client application.

Conflicts:
	app/views/layouts/login.html.erb
pass client app id through warden

Pass client_id through warden.authenticate! and then put in url querystring so that SessionsController can see it.