Feature: Allow a root element to be set on a resource.

A resource can now have a root element set.  Previously on an update
or create the method would look like:

    class Posts < Rapidash::Base
    end

    client.posts.create!({:post => {:name => "a post"}})

With a root element, the code would look like this:

    class Posts < Rapidash::Base
      root :post
    end

    client.posts.create!(:name => "a post")

The same applies to an update.