wallet/transaction/add - Adds a new transaction to a wallet.

Method parameters:
id - The ID of the wallet to add the transaction to.
amount - The amount of the transaction.
description - The description of the transaction.

Returns a JSON document of the following format:
<number> - The ID of the newly created transaction.

Expected errors:
WALLET_NOT_FOUND - A wallet with the ID doesn't exist.
ENTER_AMOUNT - The amount is zero.