Added item handling modules.
Setup base for pickling Ebay Items.
Added Types: BuyerRequirementDetails, BuyerPolicyViolations, ItemRequirements, BuyerUnpaidItemStrikes, VerifiedUserRequirements, PeriodCode.
Added picklers for many item types.
Included BuyerRequirementDetails in Item pickler.
Moved field field name out of BestOfferDetails pickler.