Support for custom REST methods

We connect to some web services which have flipped the PUT/POST methods (PUT means create, POST means update). This change allows an implementer to specify the REST method to be used for a particular action (`:create` vs `:update`) on a particular model. 

It could be expanded to support others (like `:find` or `:destroy`), but I haven't added that because I haven't needed it, and I'm not going to speculate on someone else's use case.