ID/IDREF strawman #2

Instead of <key> and <keyRef> elements, we could add key and keyRef
attributes to <data> and <anyString>

<attribute name="id">
   <data type="xsd:NMTOKEN" key="whatever"/>
</attribute>

<attribute name="ref">
   <anyString keyRef="whatever"/>
</attribute>

When anyString has a key or keyRef attribute, it would also be allowed a
whiteSpace attribute to say how whiteSpace is handled when comparing keys.

The value of the key and keyRef attributes would be a NCName identifying the
symbol space.  I think I like this better.

This would present a problem with anonymous datatypes.  I think we need to
think about those more.

James