Refactor RowTest, add coordinate class and methods, add win checking to Board.

This should have been two commits: a big test refactor and a new coordinate object w/ methods.

+ Refactor RowTest, ColumnTest, DiagonalTest to follow test case superclass pattern. (ColumnTest and DiagonalTest now subclass RowTest and make setup changes in @Before method.)
+ Add hasWin methods to Board and Row.
+ Add BoardCoordinate class to contain grid coordinates (helper methods may come later).
+ Add getter methods to Board to return square contents.
+ Change x and o fields to uppercase globally.