THREE.XHRLoader Data URI handling for Safari (OS X and iOS)

[richtr]

Safari is unable to fetch Data URIs via XMLHttpRequest. The following code throws an error:

var loader = new THREE.XHRLoader();
loader.load("data:,hello");

// works in Chrome and Firefox

// throws in Safari with: "XMLHttpRequest cannot load data:,hello. Cross origin requests are only supported for HTTP."

This PR detects whether the input URL is a Data URI and, if it is, then it will be manually processed in to the requested .responseType type, if any is provided. The default response format is String.

[mrdoob]

Thanks!