Simple Mode

This change allows you to fly a multirotor without having to worry about yaw, hence the term "simple mode". 

Before the simple mode feature will work at all though (to prevent accidental activation), you must enable it as follows:

multirotor_att_control start -s 

I have mapped channel 6/Aux1 to the simple mode function. The switch will turn the mode on/off. The switch implementation has been keep simple in light of the upcoming mode switching plans. 

For more information see the draft user guide: https://pixhawk.ethz.ch/px4/users/simple_mode