add package.json

This will allow browserify users to install three.js directly using `npm`.

The package can be installed straight from github by running `npm install mrdoob/three.js` (once this pull request is merged. for now `npm install mathisonian/three.js` works).

It would also be cool to publish the package to npm. It looks like someone [already published a browserify compatible three.js 2 years ago](https://www.npmjs.org/package/three.js), maybe we can get they will give up ownership of that package name to make space for the official one.