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
This fill fail in python3 (`filter` now returns an iterator).
You should wrap `filter()` in `list()` as you did few lines back or replace this line with something like:

    file_object = next(f for f in directory.get_files()
        if f.get_index() == index)

-- faster and more readable.