[develop 2.0] Use Gulp as build tool

This pull-request add gulp as a build tool (should address #181).

It's a start, but to be ideal, I think the following is needed:
- [ ] split each class in its own file
- [ ] make use of require for dependencies
- [ ] no more globals (see #178 )
- [ ] use npm to manage external dependencies (shred, jquery..)
- [ ] use browerify or webpack for portable browser build
- [ ] produce a bower version in the build (https://www.npmjs.com/package/gulp-bower)