Update sell-media.php

Uncommenting lines 732 - 740 is necessary in order not to prevent password protected items showing up on, for example, the Items page.
Cart should now be emptied after successful payment, #199 aka Travis
Merge branch 'dev' of github.com:graphpaperpress/sell-media into dev
Fixing issue where users did not receive their download link, #444.
updating readme
download link issue fix
login redirecting issue fix
Updating readme with login fix, #447
download list now works with reprints and legacy products, re-using functions into class, #448
Adding legacy reprints fix, added comments, removed test code, #448
fixing undefined variable
removing header no cache settings, caused php warnings
uncommented the password protected collections filter code
Filtering password protected collections from the widgets page, updating readme, #450.
Sliming down JSON object for bulk uploader, #232.

Signed-off-by: Zane M. Kolnik <zanematthew@gmail.com>
Filter added for sell media item shortcode
Default price group option added
spelling fix
Adding check for when no default price group is set, using default price group on single item page from setting, when its not set on the sinlge item, #438
updating change log