Rename top-level `sapling` to `localwiki`.

This fixes some nasty (and hard to figure out) issues with Django 1.4's
new directory structure that were caused by having a `sapling` module
inside of a `sapling` module.  This is just easier to reason about.

TODO: Explore renaming the inner `sapling` module to `core` or `main`.
      Likely not worth it right now as we're using the word sapling
      all over.