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 
Ok. Could you:

* add a comment here about why `client.hashedSecret || client.secret` is necessary
* file issues that capture the work to make the handling of `client.secret` in the code and DB better
* reference/link to those issues in the comment