lua-factory: Add support for dates in Epoch format
lua-factory: Add support for dates in Epoch format

Make it possible to pass the creation or modification dates as a number
of seconds since Epoch.

https://bugzilla.gnome.org/show_bug.cgi?id=752593

grilofw/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins