Update rspec 3 formatter to work with RSpec 3.0.0.rc1.

This should work with all 3.x releases as well -- the RSpec 3
API is locked as of the RC1 release.

This also changes the formatter to use delegation instead of
inheritance.
Merge pull request #21 from myronmarston/rspec-3-rc

Update rspec 3 formatter to work with latest rspec 3 changes.