Use broccoli-caching-writer.

Even though libsass is insanely fast, we still see broccoli-sass in the "slow-trees listing" (at around 250ms) for every rebuild regardless of what files were changed.

This change updates to inherit from broccoli-caching-writer, which simply symlinks (using symlink-or-copy package) from the last runs cache directory if the input trees have not changed.

The result of this is that when changing files in a tree other than those provided to broccoli-sass the broccoli-sass tree rebuilds are nearly instant ( less than 10ms on SSD ).

In the future, we can even restrict extraneous rebuilds further by using broccoli-caching-writer's new include/exclude patterns to prevent cache invalidation when files inside the inputTrees but not related to the sass compilation are changed.

Closes #26.