Align version available via public-repo with source

[joewiz]

This package's repo.xml file puts the current version at 0.5.4: https://github.com/eXist-db/public-xar-repo/blob/master/repo.xml (circa Oct 23, 2015).

The public-repo lists the current version at 0.5.2: http://demo.exist-db.org/exist/apps/public-repo/packages/public-repo.html (no date stamp, though this would be a nice item to track).

A new update today providing compatibility with eXist 3.1.0.

Proposal:

• Bump version to 0.6 since this change essentially provides a new feature.
• Update repo.xml with release notes drawn from d0bc5a7: Allow processor version to end with a label.
• Update repo.xml with other enhancements made before and since the last version, drawing from https://github.com/eXist-db/public-xar-repo/commits/master.
• Tag final commit as 0.6 and release 0.6 to GitHub and to the public-repo