Use equalizer instead of manually overwriting #==
Test against Ruby 2.1.0
Use descendants_tracker
Remove unnecessary scoping
Reoganize errors class
Temporarily remove memoization
Make Twitter::Place inhert directly from Twitter::Base
Missing key raises now KeyError, not ArgumentError
Make Twitter::Base objects immutable
Remove singleton from Twitter::NullObject
Freeze and memoize Twitter::Base objects with adamantium