Add -f 'fast poll' option - mitigate XMPP failure

Some are reporting a persistent problem with XMPP notifications:

https://github.com/armooo/cloudprint/issues/48 - Issue #48

The problem is persistent, and is not related to networking issues.

This is an admitted hack to provide a workaround for that case.
Invoking the '-f' command line argument causes the print queues to
be polled every 30 seconds.
Merge pull request #65 from davesteele/fast_poll

Add -f 'fast poll' option - mitigate XMPP failure