Use GST_TAG_DATE_TIME to set 'year' tag

GStreamer tagging elements only consider the GST_TAG_DATE_TIME and don't
do anything with the GST_TAG_DATE tag sound-juicer was trying to set.

This fixes https://bugzilla.gnome.org/show_bug.cgi?id=693048
Store release date as GstDateTime

The big advantage over GDate/GDateTime is that it's possible to indicate
that the month/day field is not set, which happens in metadata from
musicbrainz.

https://bugzilla.gnome.org/show_bug.cgi?id=693048
Remove unused helper

With the switch to GstDateTime, we use
gst_date_time_new_from_iso8601_string() and the
sj_metadata_helper_scan_date() helper is no longer needed.

https://bugzilla.gnome.org/show_bug.cgi?id=693048