Migrate to Webpack

[ecarriou]

We use Grunt for building System Runtime. It works pretty well but we will give a try to Webpack for:

• be part of a bigger ecosystem and
• check if we gain time and size for the build.

[ecarriou]

After some tests, the gain to use WebPack is quite limited (we gain 0,5ko in the bundle), so we will keep Grunt for now.