Use bootstrap sass gem

Removes the static bootstrap code with the standard bootstrap-sass gem.
This allows easier bootstrap upgrade, simply changing a number in Gemfile.
Another advantage is that bootstrap variables are now accesible by the main stylesheet of the site. Default bootstrap values such as gutter, etc.. can be overridden and bootstrap mixins can be reused too.

The only downside, as bootstrap-sass docs state, is that the fonts have to be manually included (not necessary if the project were Rails). This is not such a bad thing as those fonts rarely change.

It would be advisable to do this with jquery too.