verify() now checks the validity of each square, in addition to the
validity of the rows and columns.  It also checks if each nonzero value
in initialState is also in currentState.  I also spotted a logical error
in the check for the rows and the columns.  That logical error has been
corrected.

Some spelling and grammatical errors in the comments have also been
corrected.  The comments still use "nonzero" and "empty"
interchangeably, however.  I would like to change this ambiguity in the
future to make the comments more understandable.

Code has now been written for setCurrentState().

At this point, a test driver needs to be written to determine if each
method and constructor works properly.
Merge remote-tracking branch 'github/master'