Added Guard (automatic test runner) and Spring (app preloader)

Guard usage:
bundle exec guard # will watch for file changes and run related specs automatically

Spring usage:
spring command # will run the command on the preloaded environment
spring stop # stops the background server (useful when you need your next command to be run with a reloaded environment)

Issues:
Spring won't reload automatically the app successfully under some circumstances. Example:
	$ spring rspec spec/models/user_spec.rb
	$ touch app/models/user.rb
	$ spring rspec spec/models/user_spec.rb # <- Will fail, and "spring stop" is required to make it work again
Fixing this problem would make Guard significantly faster (3 secs vs 10+ secs before the spec is actually ran).