Manage dependency to Node.js in a better way

[astorije]

Switching from Node.js v0.12 to v4.x in #14 has shown heavy limitations to the current way.

Right now, it's just curling the Nodesource script, which is bad.

I tried to make this a bit nicer, by specifying which version people want, but it has so many caveats that I gave up, at least for now.

A better way could be to use existing roles, such as Nodesource's one or @geerlingguy's, but again, not ideal: Nodesource's one only support Ubuntu officially as well as Node.js < 4, and @geerlingguy's one seems to be having issues with Debian.
Note that the branch shown above is heavily inspired from these 2 roles and my own observations.

Another way to do so could be to remove Node.js installation altogether and mark it as a dependency in the README. After all, it's not really this role's job to do that... but same could be said about supervisor as well.

What about nvm? I have never used this.

Current setup works for me, but if this ever becomes an official role for The Lounge (as #12 suggests), some love is required. Anyway, any help is welcome, or needed even :-) Thanks to any passer-by!

---

EDIT: Nodesource's repo now supports Node v4 and installs it as the default.