Shepherd shouldn't auto-update major versions

[Max]

From email discussion...

We may also want to make Shepherd not auto-upgrade major version numbers. That way if we decided we really wanted Shepherd to require a new Perl module, but didn't want to break everyone's installation, we could bump from 1.x.y to 2.0 and users would continue using 1.x until they upgraded manually. This could be as simple as running it with a special option like --upgrade-version; the point is it would need to be done interactively, so users could take care of any new dependencies.

From Peter: It'd be nice if shepherd also had a switch (--check-up-to-date or some such) to list which components will not be automatically updated to their latest verions and what new dependencies are preventing the update. Ideally, this would be silent and return a 0 exit status if everything is up-to-date, So I could run it as a cron job, and get an email when (and only when) I need to install some dependencies.

[Max]

First part complete; still need to add --check-up-to-date or whatever. Also this should maybe integrate with the new pending install stuff ([288])?

[Max]

Added --pending option.