Stabilization: make sure yaw attitude always takes shortest path

There was a subtle bug in the wraparound logic for yaw control.  Both
desired and actual are specified from -180 to 180.  In the case of
desired being say -100 and actual being 100 then the error would be
-200.  After adding 180 it would be -20.  Normally modulus is defined
as being the possitive remainder, but fmodf preserves the sign of the
input so in this case instead of producing 340 it produces -20.

To correct this we just add another 360 before the modulus (which will
be removed).  This ensures all combinations of actual and desired will
result in the appropriate sign.

Fixes #464
Math: create a circular modulus function

Used to calculate equivalent angle from -180,180 from any angle
from -540 to infinity.
Stabilization: use the circular_mod function

Removes cryptic math from main code path.
Merge pull request #468 from peabody124/attitude_yaw_next

Attitude yaw next