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

[ghost]

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",
   });