[HTML5] 刮刮樂效果

http://kuro.tw/blog/2012/02/16/html5-canvas
http://www.petsbarn.com/wp-content/uploads/2012/07/java.html
http://beej.us/blog/data/html-canvas-scratcher-2/

http://www.catchmyfame.com/2011/06/28/a-jquery-canvas-scratch-off/
http://www.websanova.com/plugins/scratchpad/html5#websanova