Version renumbering

The files released yesterday were renumbered as version 0.5.1. This is because there were significant changes to Yard's design between the 0.4 release and yesterday's release, and I (MLybbert) was not responsible for them. CDiggins did those. My efforts were much more modest.

The current 0.5.0 release comes directly from Heron 0.06.01.23.