Default cache for unchanged input trees.

[rwjblue]

I have created a caching writer implementation over at rjackson/broccoli-caching-writer. It is very simple, and just compares the hash of the input tree to the last known hash, and if different calls the updateCache method. This clearly isn't the absolute best possible scenario, but it is better than no caching at all, and may be the only generic solution that we can come up with.

I would love to update this repo to use the same technique (since I see no downsides). Would you accept a PR for this?