Enabled basic cross-domain tile loading without tainting canvas

Although OpenSeadragon could already load tiles from other domains and draw them to the canvas, the canvas immediately became "tainted" even when the tile source enabled a wide-open CORS configuration because the `crossOrigin` attribute needs to be set before attempting to load the image. When a canvas is tainted, the canvas's raw image data cannot be read. By setting this attribute, however, applications can use OpenSeadragon to load image tiles from external (CORS-enabled) repositories and process them, for example, as part of an image analysis workflow.

See [MDN](https://developer.mozilla.org/en-US/docs/HTML/CORS_Enabled_Image) and the [HTML Specification](http://whatwg.org/html#attr-img-crossorigin) for more information.

Note that more sophisticated access control is possible than what I have added here, but that what I have added here already solves problems for those of us in the scientific community who curate publicly available image repositories.