Add support for cross origin

Both side of a cross origin request should be aware of it. It is done on
the client side through a crossOrigin attribute on the image.
http://stackoverflow.com/questions/12947361/cors-settings-for-images-in-canvas
https://developer.mozilla.org/en-US/docs/Web/HTML/CORS_enabled_image
Merge pull request #20 from bobylito/master

Add support for cross origin