Updated Custom item sell #4

[FEATURE] Custom items can be shipped and retrieved. (needs testing). You can only ship items you already bought from the store.
[FEATURE] Added preliminar support to itemstack serialization for temporary shipment package saving.

[BUG] Distance fee is not applied to shipped packages.

-- Code related changes
[FEATURE] RSInventory now stores all bought items for every store session. Now we can get track on every bought item until the player leaves the store. This feature was added for item shipment.
[CLASS] Added ShippedSerialization class for easy serialization to YAML of ShippedPackages object. This class helps with loading and saving yaml of ShippedPackages.