Changed mode numbers so that QUAD_SAFE is lowest number (0), first one to enter on startup.  Also, added a QUAD_CLOSELOOP state which treats stick inputs as reference pitch and roll commands, and closes a PID loop around them to track the reference physical pitch and roll based on IMU measurements.  The new mode numbering measns that the normal stabilize() function is not called when in QUAD_SAFE, QUAD_MAN, or QUAD_CLOSELOOP modes.