adding base64 support to AssetLoader

#411

Attempts to extract the file-type from the mime-type for base64-encoded URIs. The "text/plain" type looks for "txt" file extension. 

This brings up another topic. To reference the image you would need to use the same huge string. Not sure how desirable that is. Another approach would be to decouple the AssetLoader key and URI, so that you can grab a Texture (or whatever) by a short key name which has nothing to do with its URI.