store status codes ('working', etc.) as constants
tests call boolean methods rather than examine "status" field directly
add tests for status accessor methods
Merge branch 'status_codes_as_constants' of https://github.com/signaturelabs/resque-status into signaturelabs-status_codes_as_constants

Conflicts:
	lib/resque/plugins/status/hash.rb
Fix issues running the tests on Ruby 2.1