Support for namespaces in the output of odd2json.

This modification adds "ns" fields to elementSpec objects and the objects in the "attributes" array.

The resulting json file is to be interpreted like this:

1. An element with "ns" is in the namespace specified by "ns" and an element without "ns" is in the TEI namespace. (This "ns" comes from the corresponding ``elementSpec`` in the ODD or from the ``schemaSpec`` if ``elementSpec`` has no ``@ns``.)

2. An attribute with "ns" is in the namespace specified by "ns" and without namespace is in no namespace. (This comes from ``@ns`` on the corresponding ``attDef`` in the ODD. There is no propagation from anywhere else.)

I don't consider that I know everything there is to know about the ODD format so I may have missed something.