XML File Context Provider

TODO:

1. The getSchema() needs to be updated to follow the latest ontology. 2. The format of the identity data should more closely follow the Higgins

  data model and, since we control it, we can structure the XML however we
  want.  Besides, I don't like the file format as it stands anyway.  In
  addition, if we structure it right, we won't have to do any mapping of
  types from the XML to Higgins, it can just BE what we expect in Higgins or
  even the consuming application (ie. CardSpace claims).  At any rate,
  currently, that's not the case.

3. Respond to registry and configuration model changes when they are

  completed.  #2 should be done in conjunction w/ this.

4. Allow updates through IdAS APIs.