Gexml

* strict type checking for Model fields; thanks Alexander Vladimirov.

* generate fewer redundant namespace prefixes; thanks José Orlando Pereira.

* allow Model subclasses to inherit meta attributes from their base classes.
(again thanks to Alexander Vladimirov)
* allow List and Dict fields to specify a "wrapper" tag that groups the
contained items. This tag doesn't show up in the object structure, it
is added and removed transparently during rendering and parsing.

* Dict field type; thanks to Alexander Vladimirov.

* support for order-insensitive parsing
* more careful management of namespaces in fields.XmlNode.
* support for rendering fields whose attrname or tagname is a tuple
(namespace,localName)

* support for empty-only Boolean fields, where presence of the attribute
or tag indicates True and absence indicates False.