add board-analysis.js for functions that interpret the state of the board
add TOTAL constants for  loop conditionals
move get_larger() and get_smaller() to board-analysis.js
move game_over() to tic-tac-toe.js
move check_empty() to board-analysis.js
add functions to draw menu and game over text to canvas
refactored get_cursor_position() so it checks y-coordinate first
add begin_game() function
add separate event listener for the first mouse click

Before, I had the click listener handle the first mouse click by
checking a game_started variable.  I added a separate function to
handle the first click, remove the first listener, and add a
different listener for each remaining click.
remove game_started variable and begin_game() function
replace conditional in handle_first_click() to use typeof()
pass position to move_for_computer() function to eliminate last_human_move variable
add separate move_for_human() function