clean up Rakefile
added a task to test all adapters at once
enhanced sanity check when ActiveRecord is not loaded
That typically happens when using only Mongoid and disabling ActiveRecord
put some conditional code in Gemfile instead of rolify.gemspec in order to minimize dependencies
removed Mongoid RSpec matchers as they are no longer needed
added README file about instructions how to run specs
fixed the README with all changes for the next 3.3 release
added specs and upgrade instructions