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 
yes; `COALESCE` returns the first non-NULL argument, so if a new value is provided then that's used, otherwise the previous value is used and the column is unchanged.