lua-factory: allow multiple values for each key
lua-factory: allow multiple values for each key

All GrlMedia is GrlData and all its keys could have multiple values by
default from grl_data_add_* API. Till now, Lua sources were only getting
the first value of the key.

This patch fixes that by providing multiple values as an array.

https://bugzilla.gnome.org/show_bug.cgi?id=755447

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