fs: add access() and accessSync()

`fs.exists()` and `fs.existsSync()` do not follow typical node conventions. `fs.access()` and `fs.accessSync()` are added as alternatives in this commit.

This is the first part of #103. The second part is actually deprecating `exists()` and `existsSync()`. This PR is equivalent to https://github.com/joyent/node/pull/8714
So cryptic. I know I'm coming out of nowhere but what about e.g. `fs.access.EXISTS`, `fs.access.READABLE`, `fs.access.WRITABLE`, `fs.access.EXECUTABLE`.