Adding basic JSONP plugin

Adds a `d3.jsonp` function. There are certain design factors which might need tuning:

* callbacks are named `d3.jsonp.foo` in order to avoid cluttering the `window` variable with their names. but this requires jsonp implementations that support `.`s in callbacks. I think most do.
* there's no strong guarantee that this will not reuse a callback name, though there are 15^52 potentials.