MAVLink can now be imported as a proper library

 * All scripts in `pymavlink/tools` and `pymavlink/examples` can be executed with `python -m` if the mavlink repository folder is added to `PYTHONPATH`
 * More Python3 compatibility improvements
 * Updated the README to be much more comprehensive
 * Also a bugfix for having MAVLink on separate partition than the .xml dialect file in Windows (mavgen.py:138)