Postal Pigeons

Mail Receptors can now be filled and emptied with hoppers and modded pipes.

Mail Receptors now emit a redstone signal depending on how many items they contain, similar to vanilla chests.
Mail Receptors and Pigeon Coops can no longer be placed in the Nether or the End, in all other dimensions they now work correctly. Mail receptors can only be reached if they are in the same dimension as the Pigeon Coop.