Allow creating hashes for a directory based on contents

I have a large resource directory and I want to keep my hashes file small (and so fast to load).

I've implemented support for creating hashes of directories which are calculated from the hashes of their contents. Additionally, a memoizer is used to avoid hashing the same file twice.
Ultimately I wanted some granularity - I had a configuration like `src: ['images/*', 'css/**'] in conjunction with `includeDirs: true` and so the memoizer proved useful as I would get the whole directory tree for my css (minimal) and not be bloated by my images dir (where I only got the top-level dirs).

Basically, it made a difference with my configuration and so I added it...