A few more literal implementations of promisified fs.* calls.

I renamed vowfs.read and vowfs.write to vowfs.readFile and vowfs.writeFile to make them a more litteral mapping between vowfs and fs.

I added vowfs.open, read, write, and fstat. Specifically, I did this to support the needs of my module lleo/node-block-file (as I am switching from async to promises).

I tried to stick as close to your code style as possible. Even though it gave my editor (emacs w/js3-mode) conniptions :).