Install the `skinny_spec' plugin.

It's basically a set of rspec macro allowing you to write thing such as
`it_should_find_and_assign :users'.
See <http://github.com/rsl/skinny_spec/tree/master> for more informations.
It's recommended to install the `ruby2ruby' gem to have nicer error messages, but
it is not required to work.

@@ update README_FOR_DEVELOPERS accordingly.

spec/spec_helper.rb

@@ -4,6 +4,7 @@ ENV["RAILS_ENV"] = "test"
 require File.expand_path(File.dirname(__FILE__) + "/../config/environment")
 require 'spec'
 require 'spec/rails'
+require 'skinny_spec'
 
 Spec::Runner.configure do |config|
   # If you're not using ActiveRecord you should remove these