Feature: Resource method on resourceable classes

To save a user from having to define a class for every resource, they
can call the resource method to create one on the fly.

    client.resource!(:users, 1) #Makes a call to /users/1

This can be chained

    client.resource(:users, 1).resource!(:repos, 2) #Makes a call to /users/1/repos/2

This feature was requested by @mli-max in #4