Removed gems that include js libraries.

We are now using npm and bower to manage our frontend dependencies.

I also removed welcome.js.coffee since we are not using it.
Changed Gemfile to work on heroku.

I added postgres as the production database and the rails_12factor gem.
Added .buildpacks to use heroku-buildpack-multi.

heroku-buildpack-multi lets you use multiple buildpacks for your app,
see: https://github.com/ddollar/heroku-buildpack-multi

To use it, you have to run:

  heroku config:add BUILDPACK_URL=https://github.com/ddollar/heroku-buildpack-multi.git

I have added the nodejs buildpack before the ruby one, so the js
dependencies will be downloaded and installed automatically (i.e. npm
install --production ) before assets:precompile is run.
Added "bower install" as postinstall script.

This makes it possible to have the bower dependencies installed
automatically on each deploy.