Added EKF_STATE message

To facilitate plane-to-plane coordination we need to get a single state estimation for a single point in time, which we don't get from MAVLink today (as far as I can tell). Position and linear velocities for one time (well, maybe, since the sensors are read at different times) are available in one message.  Attitude and angular velocities are in another message (again, may or may not be from a single point in time).

Ideally, our autonomy algorithm folks want all of the above for a single point in time, or at least filtered so that the data represents a single point in time.

This new message is single message replacement for ATTITUDE and GLOBAL_POSITION_INT in which the state is taken from an extended Kalman filter -- it was nice of ardupilot to provide an EKF. :)