Currently (november 2012), the required packages are not part of the stable distribution; you need to fetch some stuff from the unstable ditribution.
To install the dependencies, run these commands:
$ sudo apt-get install python-{dev,pip,virtualenv} ruby-compass coffeescript npm git libpq-dev
This will install almost anything you need for development. The only thing you need to install manually is elasticsearch, which is required to run the application.
Before installing elastisearch_, you need some JRE, for example:
$ sudo apt-get install openjdk6-jre
To quickest way to install elastisearch is to simply install elasticsearchdeb.
After installing the above, create the virtualenv, as described in the README.rst
(Run the commands from the directory where you've cloned the repository.)