Tweaks for Bombs.

Made the done method virtual. Cleaned up the code for the default
done method.
Added drawSolidCircleAxis.

This method draws a solid circle with an axis line to show rotation.
THe drawSolidCircle function no longer draws the axis line.
Draw with drawSolidCircleAxis.
Added visible bombs.

A bomb shows (and counts down) for three seconds before it blows up.
Switched to a 1/r^2 method for calculating impulses imparted.