Projectile Physics

Added projectile physics: when a bullet hits a baddie it will push him.
Projectiles apply a force on the baddie at the hit location, using their
velocity to decide how much force to apply.

Removed Stopwatches from scripts because I found out about Time.time.
Time values have been changed from longs to floats, and now represent
seconds instead of milliseconds.

At projectile creation in the Weapon script, changed the applyforce call
to a simple set velocity, because for unknown reasons applyforce wasn't
working consistently on multiple machines. Force values have also been
adjusted to make up for this.

Greatly increased the universal gravity setting because it seems things
are nicer with more of it.