Add dialect option to mavloss.py.
Make scripts outside of the library include path use 'pymavlink' directly.
All internal library includes are now relative.
Remove no-longer-needed sys.path modifications.
Improve documentation for how to run included Python scripts.
pymavlink: provide a wrapper script in tools for mavgen.py
mavparse.py: fixed crc inconsistency between python2 and python3

only affected array lengths over 128, so probably a signed/unsigned
char issue