Es6-Modules proposal

I moved all the functions to es6-modules. It started as an experiment, but I think it's pretty nice.
The task `gulp modules` creates a file `compiled/underscore.string.js` which contains all the functions bundled to one file. The compiled file looks like this https://gist.github.com/stoeffel/80d470a9de841cc67f8b
The build task still creates one minified file. 
So nothing changes for the user, but in my opinion this has some big advantages:
- better structure of the project
- less merge conflicts!

What do you think about this change?