Add support for defining "base" namespace for writers

(note: moved from http://jira.codehaus.org/browse/STAXMATE-38)

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(from a suggestion on mailing list)
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It would be good to be able to define a baseline/default namespace that would be automatically used by all calls that only specify local name. This would basically just add state that defines what Namespace object to pass; currently null is always used (to mean namespace URI of ""). This would just be a short-hand notation to add convenience, not having to explicitly always pass namespace for all calls. It would apply to elements and attributes.