Make it easier to run specs

[dnagir]

This PR includes guard-rspec to run the specs continuously.

The flow for testing with this PR would be:

1. Start the guard: bundle exec guard
2. It'll run all specs.
3. Got to you spec and start adding/changing.
4. Save spec/related implementation file.
5. The spec will be executed automatically.

If you want to concentrate on a single spec/example group, just add the :focus tag and only this spec will be executed by guard:

it "should do something", :focus do
  do_something
end

Additionally the following are included:

• default .rspec config is added to make it easier to see the spec results with colours.
• Growl 1.3 notifications (Mac only for now).