Creating a quickly expiring signature

[Dan McGee]

I wanted to test behavior of an application with an expired signature,
but using `--ask-sig-expire` don't seem to be granular enough. The
minimum I can specify is either 1 day, or an absolute date (e.g.
2011-07-29), which is still 8+ hours away for me right now. Am I
missing something? Decimal values are not accepted, nor seconds,
minutes, or hours.

-Dan