Allow refresh=0 to silence all iteration output.

Pull request to resolve issue #427.  Note that this doesn't silence all output, only the iteration level output.  To verify simply run any model with refresh=0 and note that there are no updates as iterations occur.  Before it would complain that refresh=0 was invalid.

Currently we have no unit testing for CmdStan's console output so I'm not sure whether (and how much) testing is needed here.