Faster css changes

Using [guard-jekyll-plus](https://github.com/imathis/guard-jekyll-plus) as @mergulhao pointed out in #227. @aluisioazevedo tested with me and it is way better than before. Note that there are changes on the README on how to compile the css from now on.

No need to run `jekyll serve -w` and `compass watch`. Just run `bundle exec guard` and you're good to go. Pay attention only the javascript compilation process that is still the same.

If any other designer wants to test this just talk to me. But basically is just checking this branch out and then running `bundle install`