Implements Client Management Endpoints

We already had the R of CRUD, this adds the other 3. The 3 new endpoints require an oauth token with the scope `oauth`. Yo dawg, I heard you like oauth?

@rfk r?
@vladikoff you should take a look and see if there's something I've forgotten that the console will need.

fixes #60 
One day in the distant future, when FxA authentication powers a huge chunk of logins on the web and we have many millions of clients, we will want to add pagination to this API.  But not today.