new althold PID

althold should work better now, the PID settings are preliminary. There
is definetly room for improvement.
For aquiring your own PID settings set ALT_P = 0 and tune the VEL pid
until the copter only drifts a little from its position when you
activate althold. Then set ALT_P to a value where it holds the position
stable
Merge pull request #42 from luggi/betteralthold

new althold PID