Add ability to get a cached resource by url

Add mapping for resources loaded by the loader by URL and add some methods for accessing that mapping.
I used `new Image()` here because I wasn't sure that `document.createElement('a')` would work in Ejecta/CocoonJS. I'm not familiar with those platforms. If an anchor element can be used instead, this code gets a little cleaner since `_urlTemp` isn't needed anymore.