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
Maybe this could be a little more explicit about how F_OK says nothing about rwx permissions, just that the file exists.

@domenic's suggestion of having human-readable aliases (assuming he means aliases and not replacements) is not a bad one.  [FRWX]_OK is not without precedence, however; see e.g. python's os module.