Adds postmaster NPC for managing player mailboxes. Allows a single item to be sent from one player to another via the post office. Item is stored in the sender's slot & can be retrieved by receiver/target or the slot owner.

• add player "mailbox" slot
  • stores item being sent from sender (slot owner)
• create postmaster NPC
  • add logic for managing mailbox
  • charge service fee to sender
• create interior post office map
• create unit tests

Closes #737