Allowed versions of the schema

Since a few implementers have started opening their solution to final users, it's critical to determine which versions of the IDS schema can be considered canonical and valid across systems, and which will not be accepted.

Until now the auditing tool was adopting a loose approach to this, in order to accept gracefully the majority of the samples in the documentation and development folders, but I've received feedback that makes it urgent to be stricter.

I hope to be able to publish an update soon, buy beyond that we need to be documenting these matters. Particularly around the expected interpretation of `xsi:schemaLocation` for IDS files, and the contents to be published at `http://standards.buildingsmart.org/IDS`.