Board.isValid() refinement

Use log2 and pow functions to determine if tile’s value equals 3 * (2 ^
n). Added an additional unit test for isValid.
Maximum tile value is 6144

Added upper bound for tile values in Board.tilesCanMerge (two 6144’s
can’t merge because 12288’s don’t exist).
Added Board.getScore method

Added getScore method to Board. Added corresponding unit test.