Recode Packages Completely (HUGE COMMIT)

Gave packages a ton more flexibility. Fields are now as follows:

UseVaultEconomy: Whether or not to use server economy instead of DP
price: Didn't change.
description: Didn't change.
limit: Number a user can buy. 0 means infinite.
ActivateImmediately: Didn't change.
ExpireTime: Time in minutes a package lasts. 0 means infinite.
RestrictToWorlds: List of worlds this package is allowed in. Empty list
means package can be purchased anywhere.
ActivationCommands: Commands to run when package is activated. Commands
beginning with 'p:' run as the player instead of console.
ExpirationCommands; Commands to run when the package expires.
PreRequisites: List of prerequisites. Empty list means no prerequisites.
RequiredInventorySpace: Didn't change.