solo-0.2.3

diff from 0.2.2:

• bugfixes:
  • fixes a critical bug causing inertial sensor errors (https://groups.google.com/forum/#!msg/drones-discuss/fClzpedcbmo/xFC3ENo-p5IJ)
  • fixes a critical bug where RTL could attempt to fly home at 0 speed
  • fixes a land detector bug causing takeoff to fail
  • fixes an EKF bug causing the copter to report an erroneous altitude and fail to take off
  • fixes a bug causing the copter to steadily yaw
• pre-arm changes
  • sends pre-arm and arm checks at 2-second interval while disarmed
  • attempts to work around "arm: safety button" (testers: watch for copters arming but never spinning props)
  • removes arming check for HDOP
• controls changes
  • improves position control in turbulence
  • fixes certain twitches while hovering
  • autotune changes
• experience changes
  • new LED firmware
  • disables land repositioning
  • always RTLs at 10 m/s regardless of slider position