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 wat jQuery API does, 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.