Removed methods from Invasion Game

Moved these methods to the Board class since they should be implemented
there.  Moved comments as well and removed run method, which was the
thing keeping the Board class from running its own thread.
Minor Changes to Cell and Virus Class

Added a few comments and small but significant changes.  Added
withinVirus method to detect if an x and y coordinate are within the
bounds of a virus.
Big Changes to Board Class

Continued to implement key methods.  Moved some methods over from
InvasionGame.  Got the run method and thread to work and so now the
viruses animate upon start.  Currently there are just 3 viruses that
are created and animated upon initialization, however that can easily
be changed by calling introduceVirus on a timer from within the run
method’s main while loop.

Implemented methods fully to check for contact with cell walls and with
cells and both respond accordingly.  Also implemented the mouse
listeners on the viruses so if you click them they die.  Best to just
try playing a bit and you can see what I have done.