Correctly DELETE properties of nested TrackerResource instances

When a TrackerResource is serialized to SPARQL, we generate a series of DELETE {} INSERT {} statements. For every single-value properties whose value we set, we need to delete its value first. (Otherwise we might see "Unable to insert multiple values" errors).

The tracker_resource_print_sparql() function was not generating the DELETE statements for relations if there were multiple relations for a single property.

This fixes #121 (closed) and #154 (closed).