Adds Check support to the BogusGateway

The error messages for authorize, purchase, recurring, credit, and store
were previously CreditCard-centric. Now, if a check (i.e. any object
that responds to 'account_number') is given, the 'account_number' will
be used and the error message will reflect that a check is being used.

Because ActiveMerchant::Billing::Check has a 'number' attribute
in addition to 'account_number', priority is given to 'number' for
backwards computability.