some UX improvements, very hackish

hi, the first change is
 * a click on the canvas now leads to a paint, without any mousemove - feels more responsive
 * user can leave the canvas for a moment (500 ms) and come back in to continue drawing (it was kind annoying that if you wanted to draw the whole canvas, that when you accidentally left the canvas, you had to click to start drawing again)

i implemented it very hackish directly into the JS, if you want to reuse the logic you will have to write it in coffee

great lib otherwise, thx