Move audio to its own mixerscript.

Moved the flight-mode sounds to its own mixer-script.
Updated readme
Added function for getting home direction
Testing direction to home
Smaller and darker arrows

Made arrows smaller, and darker to increase visibility.
Removed artifacts from image after resize.
Added limit to max hdop value

Put a limit on max hdop value (10.2) since that's the max wither current
scaling trasfered as one byte.
Merge pull request #4 from chsw/master

Pull request from chsw/MavLink_FrSkySPort