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: f3a989c48fdc865a2482b0e368a6efab4406f4da We can replace fs.exists+fs.mkdir with mkdirp.

[jaydson]

Good stuff.