Migrate to use javax.json definition of JSON at some point

[andyjefferson]

We currently make use of org.json definition of JSONObject, JSONArray etc, but JSR0353 provides javax.json. Would make sense to migrate to this at a convenient point.

See
http://www.javabeat.net/2013/10/java-json-api-jsr-353/
http://www.javabeat.net/2013/10/json-processing-javaee-7-object-model-api/

The downside of using javax.json is that we need only 5 classes currently and these are repackaged internally. Using javax.json will require a user to have javax.json and an implementation of that API in their CLASSPATH