Added crash alert. beep beep

This feature probably wont stop many crashes but it is the first iteration of collision detection. Hopefully this can spark a conversation about serious collision detection and avoidance.
closes #1006  
@TShapinsky for the `COLLISION_DANGEROUS_SPEED` and the `COLLISION_SAFE_ALTITUDE` constant, could you specify the unit either in javadocs comments, or in the constant name.