fs: add the fs.mkdtemp() function.

This uses libuv's mkdtemp function to provide a way to create a
temporary folder, using a prefix as the path. The prefix is appended
six random characters. The callback function will receive the name
of the folder that was created.

Usage example:

fs.mkdtemp('/tmp/foo-', function(err, folder) {
    console.log(folder);
        // Prints: /tmp/foo-Tedi42
});

The fs.mkdtempSync version is also provided. Usage example:

console.log(fs.mkdtemp('/tmp/foo-'));
    // Prints: tmp/foo-Tedi42

This pull request also includes the relevant documentation changes
and tests.

PR-URL: https://github.com/nodejs/node/pull/5333


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>