Add Bootstrap

I added bootstrap which has a lot of styling that we get for free.
See http://getbootstrap.com/css/ for reference.

I modified the buttons on the welcome page with a couple css classes to make sure it was working.
The "Rails" way of doing this would be to utilize URL helpers - 

So, we'd want something like:
`link_to "Groups", groups_path, class: "btn btn-primary"`
`# => <a href="/groups" class="btn btn-primary">List all groups</a>`