Updating REST provider 

A refactoring of the REST provider that makes it possible to retrieve a service method wrapped as a middleware via `app.rest.find(service)` etc. This is necessary to implement #22 as a plugin.

Additionally the provider now also throws an [HTTP error](http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html) 405 (Method Not Allowed) if trying to access a service method that doesn't exist.