Directory Traversal

I've added a few methods to `Directory` and `File` that allow you to easily traverse up and down the directory tree:

```coffeescript
root = atom.project.getRootDirectory()

root.getFile(".config.yml")  # => File instance
root.getFile("subdir", "subsubdir", "filename.txt")  # => File instance
root.getSubdirectory("subdir")  # => Directory instance
root.getParent()  # => Directory instance
root.isRoot()  # => false, probably
```

Having these would save me a lot of unpacking and re-packing paths in and out of `Directories` to walk around the filesystem, which would be handy and clean up a lot of kind-of-messy, duplicative code. :grinning: