XML format

To support Elasticsearch in legacy environments, it would be nice to have XML as an output option in the core. For example, applying XSL stylesheets to Elasticsearch output for XML/XHTML formatting is more convenient with XML present, it avoids transforming JSON to XML.

The XML format is realized by adding another jackson dependency, jackson-dataformat-xml.

Extra XML namespaces can be added in a properties file named ``xml-namespaces.properties`` in the class path, e.g. ``dc = http://purl.org/dc/elements/1.1/``. Field names could then start with ``dc:`` to get resolved.

The JSON output is wrapped into a ``root`` element with default namespace ``http://elasticsearch.org/ns/1.0/`` and preferred prefix ``es``/