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
I think it would be better to remove the argument then. Otherwise people will be adding unnecessary if statements. For accessSync people will be even more confused, writing code like 

```js
if (fs.accessSync(...)) {
  // have access
} else {
  // don't have access
}
```

which of course will work in the happy path.