Keepalive responsetype json added

In this usecase the keepalive webservice can deal with a webservice that responds with a json message on a GET request. This response is always of the form:
{
   "id": integer,
   "result": String
}

Added the possibility to switch to using the json response. This message should contain a 'result' with the
serverResponseEquals string. So in this case the "result" will be "OK", like this:
{
   "id": 2,
   "result": "OK"
}