Correctly load the environment in the build hooks

By executing them via a login shell.
Improve a couple logging message

- Add logging when the build hooks are executed;
- Warn if the exit status of a service in `sanbdox run` is not 0.
Use "." if the path to the application is missing (closes #21)
Use the ports 2222 and 8080 instead of 22 and 80

Nothing will run as root in the containers (and so nothing will be able
to bind on a port < 1024).
Execute Supervisor via a login shell

Makes it easier to execute apps installed during the service build (e.g:
uwsgi in the Python service).
Add templating support, with Jinja2, in the builder

This will help to support more complex services such as Python. It
should also avoid further clutter from builder/services.py.
Add support for the Python service (closes #10)

And a small Django example in examples/django.