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
The problem is that there is, for example, no way to know if a file really doesn't exist or the process doesn't have permissions to access it.