add two player mode

I created a two player mode to ensure that the game, player, and board
were not dependent on the computer class. I added acceptance tests to the two
player mode because I wanted to be sure that the computer isn't active or
taking turns when two player mode is selected.
add selector for game mode

I added a selector to the index page for a game mode and added labels to
all of the existing seletors to clarify what each is doing. and cleaned
up some naming. I also updated the css to accomodate an extra selector on top.
Merge pull request #27 from zachmokahn/two_player_mode

Two player mode