Added specs to cover the [] and []= methods.
Improve comments and fix some typos.
Added each pair method and spec.
Added eql? and hash methods and their spec.
Use define_singleton_method insted of class << self and define_method.
Merge pull request #2 from carlosbrando/2.0

OpenStruct updated to be 100% compatible with the current Ruby repo. Added new methods and specs.