Create traversal methods on Directory.
Rename and generalize a bit.

Use `get`-names to be consistent with the rest of the API. Let users pass
more than one directory in at a time for convenience.
Traverse up to a parent Directory with getParent.
Traverse to parent directories too, why not!
Detect when you're at the filesystem root.
Remove unnecessary real path resolutions.
Cleaner lazy require for Directory.
Remove an unnecessary line.
It looks like it's needed after all.
Merge pull request #18 from smashwilson/traversal

Directory Traversal