Fix bank transaction status validation

This resolves Issue #182

Note that this could result in invalidating objects that previously passed validations; in particular, new bank transaction records with a non-empty status other than `AUTHORISED` or `DELETED` will now be invalid (which I believe is true to life according to the [Xero Docs](http://developer.xero.com/documentation/api/types/#BankTransactionStatuses)) where previously they were valid.