bugfix and new feature

Hello,

I'm evaluating turnstile v 0.7.0b1 for my app.  I found one bug when trying to initially set my limits via the "setup_limits" command.  This command created a TimeUnit then assigned it to limit.unit, which raised an exception.  The limit.unit setter did not accept TimeUnits as values (but instead created them based on the value passed in).  This behavior is changed so that TimeUnits are now acceptable.

I also changed the dump_limits command to accept '-' as the output file, which then prints the output to sys.stdout.

Andy