Added Bundler-support and compass-commands.

So this adds support for [Bundler](https://rubygems.org/gems/bundler) and expands `package.json`to include three new commands:

* `npm run compass-install`: Installs compass and dependencies, if ruby and bundler are present
* `npm run compass-update`: Update Gemfile.lock to reference newer versions
* `npm run compass-build`: Convert the sass-files to css-files

Why compass-build? Right now Windows does not support ";" to seperate commands in script-statements and I would like to be able to run theses commands on my windows machine.

We should consider the hint of using a `.gyp` file as build-script as presented on https://npmjs.org/doc/scripts.html