Use abstract references

Use abstract references consistently and add XSLT script to convert Donlon to the local-ref variant.

This implements the proposal of https://github.com/aixm/donlon/pull/6#issuecomment-55383035

The XSLT script can also be used for validating referential integrity. Just run the script over Donlon and search for all `xlink:href="urn:uuid:` in the result. These are the remaining, unresolvable abstract references.