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
Is anyone **opposed** to dropping the Boolean? To reiterate, failure would be indicated by an error in the callback (or thrown in the sync version), while success would be indicated by lack of an error. If no one is strongly opposed, I'll remove it tomorrow.