Added Rail gun Slugs

I've added the rail gun projectile that I wanted to add. When fired, the
10,000 kg slug will travel at over 6 km/s in the direction that the Ship
object is facing. I also made Projectile abstract.
Added Railgun class and related code

I added a class that allows Ship objects to shoot Slugs. The class that
creates the Slug has a limited supply of ammo, and will not allow
another round to be shot if none is available. Also, there is a one
minute cool down between each shot.