adding requirejs as a dependency installed via npm

Hi @paulrouget,

I've added `require.js` a a dependency to the `package.json` and remove the Git submodule. I've changed the `README.md` accordingly.

I guess this is just a temporary dependency as we will use ES6 modules in the future, right? Even if `npm` is [really popular](http://blog.npmjs.org/post/101775448305/npm-and-front-end-packaging) for front-end packages nowadays there is still a mismatch sometimes. If you look into [requirejs-bower](https://github.com/jrburke/requirejs-bower/blob/master/bower.json#L9) and [requirejs-npm](https://github.com/requirejs/requirejs-npm/blob/master/requirejs/package.json#L21) you'll see that they use different `"main"` files. This is probably wrong ([issue](https://github.com/requirejs/requirejs-npm/issues/5)). This doesn't affect us because we load `require.js` directly from the file system and not with a build step or module loading mechanism. It isn't a big deal, but you should be aware of it, especially if we want to add other dependencies in the future.