Added spec_helper and cleaned up specs. Now all specs require 'spec_helper'. Specs previously did not pass when run individually. They now do. spec_helper only requires gon, rabl, and jbuilder to ensure requires are consistent with use of the gon gem. Also, removed the error raising inside rabl and jbuilder methods as the Gon::Rabl and Gon::Jbuilder modules are no longer conditionally required.
Add rabl-rails support and tests for the new feature. gon/rabl.rb now gracefully attempts to require 'rabl' or 'rabl-rails', suppressing any LoadError raised. Specs for gon.rabl testing now use helper methods defined in spec_helper.rb to ensure either rabl or rabl-rails is loaded at any given time.
Updated README. Includes gon.rabl with rabl-rails usage details.
Merge branch 'rabl_rails' of https://github.com/jtherrell/gon into jtherrell-rabl_rails

Conflicts:
	lib/gon.rb