Allow static JSONP callbacks

RxJS now only works with JSONP responses that can be changed via querystring. There are several services out there that provide a static callback, which name can't be changed by the requester. An example of this is the realtime JSOP feed at [USGS](http://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_hour.geojsonp), which forces the program to have a callback called `eqfeed_callback`.

This patch emulates what the [jQuery API](https://github.com/jquery/jquery/blob/master/src/ajax/jsonp.js#L36) does (but only allows `jsonCallback` to be a string), and introduces a new optional `jsonpCallback` property for the `settings` parameter for `jsonpRequest` and to `jsonpRequestCold`. It doesn't change the previous behavior in any way.