Provide demo project with the application

In this pull request, added a demo/ folder which contains a demo Django project:

* a sandbox for development
* part of the test suite
* provides (tested) code samples

In order to simplify setup of a development environment, I also added a Makefile and some (buildout-powered) installation script/configuration. As a result, when you want to contribute to the project (or try the demo), simply:

* clone the repository
* `make develop`
* `make test`

A bin/demo script is provided as a shortcut to demo project's manage.py.