change images/ to assets/ and add default background image
add images for x and o
remove old first-question.png image
wrap image onload event handler in place_image() function
add functions to get coordinates from specified position

Previously, I had big switch statements to map positions to
coordinates.  Since the numbering of the positions goes in order
around the board, I used a sin function to map the position
to the upper left corner of the box.
edit x and o images to blend in with background more
finish functions to get coordinates from position

I updated the place_image() function to take the position as a
parameter, and then figure out where the image should be placed
from that.  It figures this out with get_x/y_from_position functions.
These check if the position is 8 (the center), and then calculate
the coordinates if it isn't.
remove unnecessary draw functions