For v1.1.1

This change set features:

* Documentation changes.
* Marking two attributes for removal: `creator/@version`, and `hashdigest/@base`.
* Relaxing definition of `<uid>` and `<gid>` to allow for string content, primarily for NTFS.
* Allow attaching an `<error>` to a `<volume>` to indicate file system-level parsing errors.
* A bug fix for specifying the number of children of the `<metadata>` element.
* Allowing non-Dublin Core elements under the `<metadata>` element.

I think this change set has low potential for controversy, so let us give this 10 days (since the [initial pull request](https://github.com/dfxml-working-group/dfxml_schema/pull/21)) for any discussion. I will merge the changes and update the version tag and text on the 31st.