Allow changing the URL query string parameter name used to specify the callback function name

We are calling our API which allows querying on any field, as such we prepend control parameters with an underscore to avoid field name collisions, ie: _callback, _page_size, _page_num. This patch allows us to use Rickshaw as follows:

        var graph = new Rickshaw.Graph.JSONP({
            element: document.querySelector('#chart'),
            dataURL: "{% url 'api:dataset-data' dataset.namespace.slug dataset.slug %}",
            callbackParameter: "_callback",
       });