fix component support

I'd like to use d3 as a [component](https://github.com/component/component). @visionmedia has created [component/d3](https://github.com/component/d3), but it is out of date, and generally keeping it up to date is non-sustainable since it involves human intervention. So let's make this repo a component.

I noticed there is a [component.json](https://github.com/mbostock/d3/blob/master/component.json) file in the repo, but it wont work since a couple things are missing. Particularly:

* `main` needs to be in `scripts` also. (see [spec](https://github.com/component/component/wiki/Spec#main))
* `d3` needs to be exported in [d3.js](https://github.com/mbostock/d3/blob/master/d3.js) so that it can be `require()`ed later.

This PR fixes the issues mentioned above, but it might be missing a few things.

* I'm not sure about the [globals.js](https://github.com/mbostock/d3/blob/master/globals.js)
* the `module.exports` added detection at the bottom of [d3.js](https://github.com/danmilon/d3/blob/bc9932af96fbe8d88655849705cd5e40588c2e59/d3.js#L8440-L8442) might not be desired

Suggestions are welcome.