Added feed forward gain to roll when in hover

Added feed forward gain that adds in right aileron proportionally based
on throttle setting when hover_flag is true.  Will hopefully help roll
oscillation problems.
Added offset to feed forward gain

Added a throttle offset to aileron feed forward gain.  During tests it
looked like roll oscillations were still happening at low throttle
settings.  By adding a throttle offset of 25 and keeping the FF gain at
20, the aileron deflection is increased by 5 degrees for all throttle
values.