Fixed equipment.Drill.mine() method

Changed mine() method to check for Cells that have run dry.  Also made
it return the total value of helium-3 extracted and updated the Javadoc.
Wrote Shield class (not quite finished).

Wrote almost all of the Shield class.  Only thing incomplete is the
defend(Weapon sword) method, which depends on writing the Laser class.
Also changed Vehicle's Equipment variables to final and added a helium-3
int.
Renamed Shield's defend method to blocks

Also updated Javadoc.