fs: add access() and accessSync()

fs.exists() and fs.existsSync() do not follow the typical
error first callback convention. access() and accessSync()
are added as alternatives in this commit.

Fixes: https://github.com/joyent/node/pull/8714
PR-URL: https://github.com/iojs/io.js/pull/114
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Domenic Denicola <domenic@domenicdenicola.com>