Upgrade to RSpec 3

Hi dblock,

this is a (non-downward-compatible) PR for RSpec 3 using [rspec-legacy_formatters](https://github.com/rspec/rspec-legacy_formatters)
One could probably maintain RSpec 2 compatibility with some version magic, but I don't know if it's worth the trouble...
Also, here is an example of how to really upgrade formatters to RSpec 3 style:
https://github.com/bsingr/growl-rspec/commit/4a82ff9213b1b85795a5a279b0831d6af76948d1

Hope this is useful :)
  Martin