AC_WPNav: rename get_horizontal_velocity to get_speed_xy

This new method name is consistent with the equivalent in the
AC_PosControl class
Copter: integrate AC_WPNav get_speed_xy name change
AC_PosControl: set_speed_z accepts positive descent speeds
Copter: init vert speed and accel for each flight mode

This resolves issue #1021 in which LAND mode could descend at the
PILOT_VELZ rate instead of the WPNAV_SPEED_DN

Pilot defined acceleration is used for AltHold, AutoTune , Circle,
Hybrid, Loiter, OF_Loiter and Sport flight modes
Waypoint Nav (ie. AutoPilot) acceleration is used for Auto, Land, RTL
AC_WPNav: add WPNAV_ACCEL_Z

Allows configurable z-axis acceleration during missions
Copter: add PILOT_ACCEL_Z parameter

Allows configurable z-axis acceleration when pilot is controlling
altitude
AutoTest: print failed copter test

Name of the failed tests appears at the moment it fails and then again
after all tests have been run.  This hopefully makes it slightly easier
to know which test has failed
Copter: init pos_control z-axis after setting speed and accel
AC_WPNav: init_loiter sets speed, accel before calcing stopping distance

The stopping distance depends upon the speed and acceleration so these
must be updated first
AC_PosControl: add set_target_to_stopping_point_xy method

Fixed bug in get_stopping_point_xy in which it would update Z-axis
target if vehicle was moving less than 10cm/s horizontally
Copter: disable RALLY points by default

This saves 2k of flash which allows the code to fix on the APM1/2