Internally cache blocks rather than raw resource values
Stop reporting directory itself when enumerating
Remove URL subclass docs

They're no longer true; we're swizzling rather than using a custom class
Have a decent stab at guessing NSURLParentDirectoryURLKey
nil can be used to trigger swizzling logic

Private implementation detail for now though
Fill in parent directory info when resolving relative paths
Canonicalize URLs to include username for home directory caching too
Make home directory store threadsafe