Allow fs functions to be called without fs as this arg

[tschaub]

The node fs functions can be called without the fs module as the this argument. This is fairly common practice with things like async:

async.map(paths, fs.stat, function(err, results) {
  ...
});

With this change, the mock stat, mkdir, and readFile functions can be called without the mock itself as the this arg.

[tschaub]

I reordered the methods so the async versions are defined first. I recognize this makes the diff bigger than it has to be. I can rework the commit to minimize the diff if it would make review easier.

[tschaub]

In case anybody else runs into this issue, I've created an alternative: mock-fs. This works with Node 0.8, 0.9, 0.10, and 0.11 (as of this writing) on OSX, Linux, and Windows.