Add title attr_accessor on Jobs

The title is how print jobs are listed as in the print queue.  This allows setting it
to something useful for identifying jobs in case they need to be
paused/canceled

I'm writing a thin REST api service on top of cups and using your library to submit and list the status of jobs.  The only problem I've ran into was that all the jobs appeared in the queue as "rCups".  This should fix that.

Let me know if you'd like it reworked in some way, I'd be happy to do so.