Tested and added empty_space? method.
move_distance_calculation tests pass - on to valid move test
Added migration, tests and seeding for the unique_piece_id into the Board class.
Pulled row and col validations out of .valid_move? method into their own separate methods:
  .valid_row_dist? and .valid_col_dist

Added method to determine player direction in positive or negative values.

Added ability for .move_direction_calculation method to determin positive and negative distance as well as validate that the distance can only be 1 and 2 squares.

Added tests for all of the methods I added and made the .valid_move? method pass all tests Greg wrote.
Fixed a mistake in the migrations, added tests for the opponent_in_jump_midpoint method, but I can't get them to work. Added models for the test.
Merge pull request #17 from heaphyg/game_model_tests

Game model tests