Add support for RSpec 3

Hi, with RSpec 3 they changed the API for Formatters. I've added support for RSpec 3 here.

I copied the tests for rspec_2 into a rspec_3 and then wrote a formatter using the new api. The tests all pass. I didn't quite understand what was going on with the "ancestors" test, but they work so hopefully I did that correctly. 

I also tested it by building and installing the gem and running it against a project and it worked great.

Let me know if I missed anything.