:lipstick: Add "repository" and "contributors"

Improve package.json by adding "repository" field, changing "author" to
the organization details and moving current author in contributors, and
adding the atom-shell version that we rely on.
:bulb: Improve Grunt tasks

Make build tasks more consistent across platforms, rebuild atom-shell
rather than hacking in different files to replace branding.
:book: Document new Grunt tasks