Add a Sinatra/Padrino notice for use

- following #486
RDoc styles...
Add Rubinius gems to Gemfiles.  Update Rakefile to handle Rubinius/Rails 4 case where RDocTask is not available.
Merge pull request #489 from petergoldstein/feature/rbx_green

Get specs running under Rubinius
rbx x AR edge is the only one known failure now