UPDATE: p-cdn.com reference added to fix abrupt playlist exits

* adds p-cdn.com as an alternate source URL for songs
* fixes abrupt exits from the addon when song source changes
UPDATE: update to most recent upstream 'pithos'

* 'pithos' updated from repo [2b35f36b91 on 2013-12-19]
* minor changes to script to correspond to newest 'pithos' API

* NOTE: 'pithos' repo @ https://github.com/pithos/pithos