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 
Given the high value of this as an attack target, it may be worth adding explicit tests for failing auth scenarios, e.g. missing auth, auth with wrong scope, etc.  IIRC we have some of these tests on other endpoints so I'll be very surprised if this is not working as expected, but paranoia doesn't seem out of place here.