Add main property to package.json.

Fixes #340.

package.json

@@ -19,6 +19,7 @@
     "type": "git",
     "url": "http://github.com/mbostock/d3.git"
   },
+  "main": "d3.js",
   "dependencies": {
     "uglify-js": "1.1.1",
     "jsdom": "0.2.8",

src/package.js

@@ -9,6 +9,7 @@ require("util").puts(JSON.stringify({
   "homepage": "http://mbostock.github.com/d3/",
   "author": {"name": "Mike Bostock", "url": "http://bost.ocks.org/mike"},
   "repository": {"type": "git", "url": "http://github.com/mbostock/d3.git"},
+  "main": "d3.js",
   "dependencies": {
     "uglify-js": "1.1.1",
     "jsdom": "0.2.8",