Add image support for feed items.
Update item image support to conform to the RSS spec.

Instead of an image section that is not allowed, this uses the enclosures as is expected. If image size and mime type are not passed, but image url is, we only use the itunes image tag. This can be desirable when we want the itunes tags, but no enclosures as some podcast apps show enclosures as downloads.
Fall back to only supporting the itunes namespaced image on feed items.

The spec is unclear on multiple enclosures, and most feed readers seem to only expect one.
Merge pull request #7 from isaacgrant/item-artwork

Add image support for feed items.