Update to 1.6

* Feature: Added option to set a given price group as the site default
* Tweak: Password protected collections are filtered from the widgets page and no longer show on the front end
* Tweak: Added filter `sell_media_purchase_text`
* Bug: Customers are no longer redirected to the WordPress dashboard after login
* Bug: Addressed issue when some users did not receive the download link
* Bug: Addressed issue where cart was not emptied, due to session not being started and browser cache


git-svn-id: https://plugins.svn.wordpress.org/sell-media/trunk@769853 b8457f37-d9ea-0310-8a92-e5e31aec5664