Use JSON for jammr REST API

Note: this pull request is based on my qt5-port pull request.

The jammr REST API has been available both in XML and JSON.  Since Qt5
added support for JSON we can switch away from XML.

The XML parser classes in Qt are clumsier than the JSON parser.  The XML
schema was also ugly because it simply wrapped JSON objects.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>