Make the code a proper module and command

This commit applies several changes in order to turn the
renderer code into a proper python module and command that
can be easily installed using the setuptools module.

Main changes:
- Main executable is named atc
- Use setuptools to install renderer executable
- Improve tests so they can be easily executed by packages
- Update README instructions

Signed-off-by: Luis Araujo <luis.araujo@collabora.co.uk>