Added filter to img src so you can filter to use custom promo images instead of regular featured image, minified CSS, added link to Docs, tested for WP 3.6.1 compatibility.

git-svn-id: http://plugins.svn.wordpress.org/easy-digital-downloads-related-downloads/trunk@771256 b8457f37-d9ea-0310-8a92-e5e31aec5664
Merge branch 'master' of https://github.com/isabelc/EDD-Related-Downloads