Framework for testing

make-release now runs tests before creating the new release.  This can
be skipped with the -s flag.

The current user must be able to create new postgres databases; the
tests create a database "quicktill-test" temporarily.
Prevent card payments before card machine rollover

Closes #53 on github.

Configure this by adding a rollover_guard_time argument to the
CardPayment payment method in the pub config file.