Django-mptt

Fixes missing static files (javascript, CSS) in the build.

- Fix issues with `raw_id_fields` in admin.
- New `DraggableMPTTAdmin` admin class that provides drag & drop node moving functionality

New features:
- Add support for python 3.5 (dropped 2.6)
- Add support for django 1.9 (dropped 1.4, 1.6, 1.7)
- Add a node_moved signal
- Spanish (Argentina) translation
- `TreeQuerySet.get_cached_trees()`

Fixes etc:
- Much faster `get_queryset_ancestors/descendants`
- Fixed 176 (infinite recursion when using `.only()` / `.defer()`)
- Much improved test coverage