Added support for type attribute on sources

…works with empty type, image/svg+xml, and image/webp. See
http://www.w3.org/community/respimg/2012/08/21/thoughts-on-adding-a-type-attribute-2/

Not sure if this is something you'd want to add, since it's not officially part of the recommendation yet, but it'll certainly be useful for people trying to deliver vector graphics to devices that support them.