Descriptive commenting of InvasionGame class

Some descriptive commenting of InvasionGame class.
Changed the name of the "screens" variable to "gameScreens" to better
describe that the flippable screens
More descriptive commenting
Cell/Virus Classes and Board Class framework

Implemented Cell and Virus classes fully.  Implemented some basic frame
working for board class, including drawing the cells and virus, and
animating the virus.  ISSUES: Cannot get the thread to run for the
board class.  Not sure why, but the run method is just never getting
called for board and so cycle nor animate is ever getting called.
Virus Class

Virus Class fully implemented
Virus/Cell Classes
Cleaning up Code
Merge branch 'master' into Alex's-Branch