Feature: RSpec 3 support

The RSpec 3 formatter currently replaces the RSpec 2 formatter.

The class is called RSpecFormatter and aliased to RSpec because the
following error is raised when naming the class RSpecFormatter directly.

    rspec.rb:53:in `<module:Reporter>': superclass mismatch for class RSpec (TypeError)

RSpec 3 has completely overhauled the formatter system using a
notification system that allows developers to optionally register
interest in particular notifications instead of implementing them all.

Please read:

  http://myronmars.to/n/dev-blog/2013/07/the-plan-for-rspec-3 for more information.