solves #67: massive refactor to use ES6 modules under the hood

complete refactor to align with all other `intl-*` projects that are also using ES6 under the hood.

here is the list of things (from the top of my head):

* `dist/` folder now host the distro for browsers
* `locale-date/` remains the same, holding the computed data from cldr, including a new `complete.js` used to produce `dist/Intl.complete.js`
* `src/` is the source of true, where you put your ES6 modules.
* `src/core.js` is the former `Intl.js` from root
* `src/exp.js` is an example of how to extract pieces from core into its own module (hopefully we will have more and more of those in the future, since that's the whole point of this PR)
* `src/main.js` is for browsers only, a trick to expose the polyfill as global `Intl` when there is no `Intl`.
* `index.js` is the entry point for nodejs when people do `require('intl')`, and it does the detection, it expose `Intl` as global when needed and set the complete locale data in memory as well.
* `lib/` folder is a 1-1 mapping of `src/` for commonjs/nodejs, and `index.js` uses it when you require intl.

Other considerations to keep in mind:

* `dist/` folder is committed so we can use those thru bower :/
* I haven't test the output in a browser, but it does work in node.js
* the entry in `package.json` called `jsnext:main` is a new feature that we are promoting, and it is what the grunt task uses to transpile the ES6 modules (we can talk more about that)
* I'm proposing to make this `1.0.0` since the code is stable (I didn't change anything in the logic of the polyfill)

Development:

I tried to keep everything the same:

*  no more `npm run build`.
* `grunt cldr` and it produces the `locale-data` folder, it does not touch anything outside of the folder from now on.
* no more `npm run lint`, `npm test` does that now. `grunt jshint` is also available.
* `grunt` alone does the build process, which includes cleaning, linting and building `dist/` and `lib/` folder.
* `npm test` does not build `cldr`, nor the `dist/` and 'lib/` folder, just like it was before, but it now runs a clean up process and linting.

alright, makes sense? happy hacking!
We are in one of them. It's worth looking at the ES6 spec. to see if `default` is spec'd to be enumerable. But the non-writable part makes sense to me for this.