FIX: D3.Events now inherit from the lib.d.ts Event interface.

Addresses #2478. By [D3's documentation](https://github.com/mbostock/d3/wiki/Selections#d3_event), "the d3.event object is a DOM event". As such, I made D3's interface extend from the lib.d.js Event interface. I had to rename Event to D3Event in order to do this.

Should be a quick fix! Let me know if there is a problem!