closes #57. refactored targeting calculations for kicking

The Aim behavior now doesnt do target calculations, you simply give it
a Point and it does its thing

The calculation of a single Point to aim at given a target Segment now
takes place in _Kick, which is the abstract superclass of PivotKick and
LineKick

PivotKick recalculates its target point repeatedly, while LineKick
recalculates its target point once at the beginning before lining up