Add the ability to retrieve the list of transactions.

The new get_transaction_register() function returns a list of
objects representing transactions. Each one looks something like:

  { "date": datetime.date(2013-11-12),
    "payee": "Payee Name",
    "accounts": [
      { "account": "Account:Name", "amount": "$10" },
      { "account": "Other:Account", "amount": "$-10" }
    ]
  }

Closes #5