AP_AHRS: added AHRS_GPS_MINSATS option

if the number of visible satellites is below AHRS_GPS_MINSATS then
don't use the GPS for acceleration correction for attitude
AHRS: perf improvement by caching declination vector
AP_Math: added mulXY() for matrix3 and vector3

returns the XY components of the product
AP_Math: implement vector2 cross product

useful in DCM for faster yaw correction
AP_AHRS: optimise yaw drift correction

use new vector2 cross product, and factor out the z component of a dcm
mul_transpose() to reduce the number of floating point operations for
a yaw drift correction cycle
Copter: remove unused radius_of_earth constant
AP_Math: use DEG_TO_RAD in longitude_scale

Also increased accuracy of RadiansToCentiDegrees although it is like the
compiler will throw away the extra digits anyway.
InertialNav: use AP_Math's longitude_scale
Copter: disable centrifugal force correction when disarmed
AP_L1_Control: Cleaned up calculation of damping to reduce computations

also removed _cross2D, using math library instead
APM_Control: Added tuning guide readme
APM_Control : Removed 20Hz low-pass filters on rate gyro inputs.

These are not required due to MPU 6000 filtering.
APM_Control: APM tuning guide.txt - Added step by step tuning description.
APM_Control: APM tuning guide.txt - Minor editorial corrections
APM_Control: Added alternate tuning method for roll and pitch loops

The alternate tuning method for roll and pitch will give a better
results, but comes with more risk as it will result in a larger rate
feedback gain
APM_Control: APM tuning guide.txt - Updated tuning instructions with numerical values for alternate method
APM_Control: Modified calculation of bank compensation rate offset to prevent climb if speed > max fbw speed

The previous calculation constrained the speed used to calculate the
bank compensation rate offset between the min and max fbw speeds. This
would result in an unwanted climb if flown above the max fbw speed
(this could happen in fbw-a mode)
Plane: use new APM_Control API
Plane: rename APM_Control parameters

this groups the parameters more logically
Plane: Fixed bug in groundspeed_undershoot that could cause a fly-away in windy conditions.

Use the component of velocity in the forward direction rather than the magnitude