Relax UID/GID type definition for NTFS

One DFXML generator outputs NTFS SIDs for the uid element:
<https://github.com/Sebastienbr/DFXML-EnCase/blob/master/DFXML.EnScript#L743>

For NTFS, it would be appropriate, and infinitely more useful, to allow
<uid> to be a string.  For other file systems, the usual numeric
constraint should still be noted.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Add a README

This duplicates the information in the schema header, with some slight
reformatting.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Document <id> role in light of other id mechanisms

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Mark "version" attribute on "creator" element for removal

This attribute appears to be unused in all current tooling, and only
provides redundant information.

This addresses Issue 2:
<https://github.com/dfxml-working-group/dfxml_schema/issues/2>

Acked-by: Mark Matienzo <mark.matienzo@gmail.com>
Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Clarify dual purposes of library element

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Deprecate hashdigest/@base attribute

After initial reporting, my conclusion is that this attribute is
vestigial.  If somebody has a need to write hashes in a base other than
16, we can restore this attribute, but so far I have not come across
such a need, inspecting the source of the DFXML C++ library, PhotoRec,
Hashdeep, and Fiwalk.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Clarify specification of parent_object

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Add error reporting to volumes

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Allow metadata to have multiple children

The defaults of the metadata child specifications only allowed 0 or 1
elements.  This was accidental.

This commit resolves Issue 17:
<https://github.com/dfxml-working-group/dfxml_schema/issues/17>

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>