extra/fixtures: add XML fixtures for address book entries

These entries contain only fictional people and
organisations.
add serialize.py to create XML fixtures

This script creates XML fixtures for the given list of
models from `cams' and `mrwf.extra'.  It can be run this
way:

  ./manage shell
  >>> import serialize

Then copy all *.xml to extra/fixtures/.  To import them:

  ./manage loaddata people organisations contactables contacts