Update unstable with current master branch
Clarify remaining maximum occurrences

I didn't realize that the default for an unspecified maxOccurs is 1,
not "unbounded."  This rendered the use of "##other"-namespace elements
limited to a clearly incorrect degree.

This patch clarifies all remaining min/max element occurrences, found
with:

    grep minOccurs dfxml.xsd | grep -v maxOccurs

One is left unspecified: the upper bound on <byte_run> elements is in a
proposed revision here:
<https://github.com/dfxml-working-group/dfxml_schema/issues/5>.
Temp

Got a better idea for how to do this
Allow other-namespace attributes on all DFXML elements

Somebody asked if the all elements could be made extensible:
<https://github.com/dfxml-working-group/dfxml_schema/pull/9#issuecomment-25035429>

In response, this patch defines wrapper types for all the basic "xs:"
types DFXML elements use, adding an anyAttribute extension.  Now any
element in DFXML can have an attribute from other namespaces.

One extra change mixed in: partition_offset was a positiveInteger type.
It's legal for the offset to be 0, so this was changed to
nonNegativeInteger.