Remove the dependency on the external `tree' shell command

- Augmented Tree now comes with a built-in tree generator which is
  mainly constituted by `aug-traverse-dir' which produces the
  internal representation (hash table) and
  `aug-pretty-print-hash-table' which renders the unaugmented tree.
- `aug-generate-tree-string' is the main entry point for the tree
  generateion. It also kicks off custom sorting.
- All means necessary for hanlding an external `tree' command are
  still included.
- The built-in tree generator is, of course, slower than an
  external command. This is due to the nature of Elisp (vs. C),
  but also due to bad design. This is acceptable, though, because
  Augmented Tree is for individual project trees, not for complex
  file system structures.