Add python3 support

This adds python 3 support, but also makes minor changes

- README rewritten using markdown
- [PEP8](https://www.python.org/dev/peps/pep-0008/) compliant formatting

This requires the python3 branch of openant (https://github.com/Tigge/openant/pull/4)

Tested using antfs-cli on python 2.7.8 and python 3.4.1 with the Garmin Forerunner 610
Is absolute_import actually used?