Speed up TDD cycle with Guard and Spork

Guard automates running the tests. More info
about Guard can be found here:
https://github.com/guard/guard

Spork loads the environment once, and then
maintains a pool of processes for running
future tests. Spork is useful when combined
with Guard. More info on Spork can be found
here: https://github.com/sporkrb/spork-rails