WPNav: add acceleration parameter

WP_ACCEL added to allow user control of acceleration during missions.
Loiter acceleration made to be half of loiter max speed
Copter: update AC3.0-rc6 version and release notes
Copter: reduce default AltHold P to 1.0 (was 2.0)

This helps reduce jumpiness due to althold feed forward
Copter: add @Increment to PID parameters descriptions
Copter: update firmware version to 3.0.0
PX4: Remove -Os because I've changed the px4 tree to have that as the
default
AP_Baro: TEMP parameter comment fix
autotest: fixed pymavlink tools paths
autotest: fixed handling pf pymavlink paths
Copter: update FS_THR_VALUE parameter description

One user was setting FS_THR_VALUE too low, below 900 which means
ArduCopter would not trigger when the ppmencoder pulled ch3 low after
updates from the ppmencoder stopped
Rover: update FS_THR_VALUE parameter description
Plane: update THR_FS_VALUE parameter description
autotest: fixed import of mavwp
autotest: added $HOME/APM to PYTHONPATH
autotest: update timeout again

we're getting too close
Copter: show pilot input in dataflash ATT message even when in AUTO

AUTO mode ignores pilot input but we still want to be able to see the
pilot's input in the Roll-In and Pitch-In columns of the dataflash's ATT
message
Copter: increase default CIRCLE_RATE to 20deg/sec
Plane: param description change for RC5 ~ RC11

Reversing the order may avoid duplication of param descriptions in the
auto generated parameter web page
APM_Control: Pitch Control - Reduced the speed below which the integrator is locked

This is to allow for slow speed flare manoeuvres in FBW modes
APM_Control: Added integrator limiting adjustable by an advanced user parameter

this is compatible with the old IMAX settings