Explicits usage of sum from math package

Avoids conflicting names
Adds 3 more mock tags
Adds 2 more mock tags
Adds debug messages
FIX buggy list iteration for one-element list
FIX conflicting var/func names
FIX playlist cleaning erasing itself
Adds play_order field to tracks

Adds prints to debug annoying "TypeError: <Response 207 bytes [200 OK]> is not JSON serializable"
occuring when flushing new Playlist entry to database
FIX issue with json (previous commit)
FIX "TypeError: datetime.datetime(2013, 5, 8, 23, 37, 3, 279124) is not JSON serializable"

The list of playlists is now fetchable! :)
Minor update