Use Haml instead of ERB in views

As discussed in issue #9, the template could very well use Haml instead of ERB. Don't know if there is any consensus on this part, but here you go. :)

This is my first time editing a Rails template, as well as my first pull request on GitHub. I think I covered everything, but have a look around and try it out.

I also made a separate branch where the download link is changed (lime@4b695dc) , so if you want to try out the Haml version immediately you can run:

    rails new my_haml_app --skip-bundle -m https://raw.github.com/lime/rails4-bootstrap-devise-cancan-omniauth/download/rails4-bootstrap.rb