DRY and Validate

In response to toji's extension drafting experience, I have:

1. Removed the list of extensions from the Makefile
2. Added a new find-exts script that uses "find" to discover the extensions based on subdirectories with "extension.xml" files
3. Added ISO Schematron XSLT system
4. Added an ISO Schematron schema to check extension stage against @href against name
5. Brought all existing extensions into conformance with the schema
6. Regenerated the registry