Add cooldown percentage method to attacking unit

For the moment, this is only useful to help me determine how much of the
timing arc to draw around a unit. It might prove useful in other
circumstances later.
Draw cooldown timer loop on every unit

The length is based on the time elapsed since the units last attack. At
the moment, everyone has the same attack speed, and range isn't factored
into an attack, so everyone's loop are exactly in sync with eachother.
Slow Archer's AS to show different cooldown loops
Move units toward target based on range, not width
Consider unit range when trying to attack

Instead of only consulting cooldown, unit must also make sure target is
in range before attacking.