NEW FEATURE: Subscribe to podcast widget, adds subscribe on iTunes and via RSS links to side bar, with optional link to subscribe to podcast page.

* iTunes image requirements updated, 1400x1400 minimum size is now "required" and 2048x2048 is now the maximum size for iTunes artwork.

git-svn-id: https://plugins.svn.wordpress.org/powerpress/trunk@993569 b8457f37-d9ea-0310-8a92-e5e31aec5664