GraphSailConnection: Tracking the previous subject since data will often list relations for the same subject consecutively.
GrailSail: addVertex method, helpful for adding vertices before adding relations to save many initial index lookup misses.
Merge branch 'GraphSail-addVertex' of github.com:juharris/blueprints
Merge branch 'GraphSailConnection-track_prev_subject' of github.com:juharris/blueprints