Feature/cucumber support

This pull request adds support for using puffing-billy in Cucumber tests, addresses #5. Specifically it adds cucumber as a dependency and adds a setup file. This PR is probably a starting point.

Things that this PR might want to do:
* turn off hard runtime require'ing of both cucumber and rspec, instead use either if they can be required
* refactor the lib/billy/{cucumber,rspec}.rb files into a common file, there is significant overlap
* Add a demo to the README for cuke integration (see my example branch in the repo).