put move_for_human() in tic-tac-toe.js
refactor ai.js so it doesn't call functions from draw-functions.js

In order to prevent ai.js from calling functions from draw-functions.js,
I basically just needed to change calls to "put_o_in()" to return
statements.  Each function (try_to_win(), try_to_block, and
win_or_block()) returns -1 if it couldn't find a move that wins/blocks.
The end result is that ai.js returns a number to tic-tac-toe.js, which
calls the drawing functions.
refactor try_to_win() and try_to_block()

Both try_to_win() and try_to_block() were analyzing the board to
decide the move, so I moved that logic into board-analysis.js in
functions called human/computer_can_win().  They return the index
of the winning combination, or -1 if they can't win.
remove unnecessary blocked variable