Parse Version query parameter for XML namespace

See issue https://github.com/adamw/elasticmq/issues/3 that explains why I'm interested in these changes ;0

- Base response off http://queue.amazonaws.com/doc/%s/ where %s
   is supplied by the Version= query parameter
   Default to 2012-11-05, but other standard values are 2011-10-01,
   and 2009-02-01