Adds a metaclass method to all Ruby objects.

Motivations :-

* Even though WhyTheLuckyStiff's [metaid gem]
  (https://rubygems.org/gems/metaid) does something similar, apparently the
  metaclass method without underscores [doesn't play well with Rails v2.3]

* I'm trying to extract code out of the [mocha gem]
  (https://github.com/floehopper/mocha) and this is an obvious candidate.