Remove all fs.exists/fs.existsSync calls

[UltCombo]

Those methods are deprecated.

We can either rewrite the logic to don't need those checks or replace them with fs.access. The latter will require a polyfill if we want to keep compatibility with Node.js 0.10.

[UltCombo]

Making progress: f3a989c
We can replace fs.exists+fs.mkdir with mkdirp.

[jaydson]

Good stuff.