This property is displayed in the XML and JSON serialization of a job item.
It is also backuped and restored from a UWS backup file.

/!\ WARNING: Small modification of the API! Since creationTime must be
             set by the UWS service at creation of a job, this date must also
             be imported at backup restoration. Hence the modification of the
             interface uws.service.UWSFactory and its implementations
             (particularly tap.AbstractTAPFactory). Similarly the class
             uws.job.UWSJob and tap.job.TAPJob have underwent exactly the same
             modification in one of their constructors.

This commit also adds the new optional jobRef items: creationTime, runID and
ownerID in the XML and JSON serializations.