Add Node support via xmlhttprequest.

[mbostock]

Related d3/d3#2736, we could use xmlhttprequest to add support for Node. Though, I’m not sure yet exactly how we express that dependency for Rollup so that it doesn’t affect non-CommonJS environments and Browserify.

[mbostock]

This seemed to work:

var Xhr = typeof XMLHttpRequest === "function"
    ? XMLHttpRequest
    : require("xmlhttprequest").XMLHttpRequest;

Though, d3.html doesn’t work, since it would also require a jsdom dependency and a global document.