Fix behavior of Twitter::Tweet.entities? method.

Previously, this method returned true when inflating a
tweet that has no entities because the inflated model
does not set @attrs[:entities] to nil, but to a hash of
empty lists.
Merge pull request #453 from michaelherold/entities-fix

Fix behavior of Twitter::Tweet.entities? method.
Cleanup #453
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
Convert if/else to use ternary operator
Fix warning URI.escape is obsolete
Dynamically define URI predicate method aliases
Refactor Twitter::Base.define_attribute_method
Follow parameter issue fixed in the Twitter API
Give methods more natural names