Fixed ImportError in `admin_tree` occurring in current Django `master` 

Fix typo in MP_Node.fix_tree doc

Merged in nnyby/django-treebeard/nnyby/fix-typo-in-mp_tree-docs-databse-databa-1429035855478 (pull request #20)

Fix typo in mp_tree docs: databse -> database

Calling delete() on a MTI inherited model can leave orphans

--HG--
branch : fix-typo

--HG--
branch : nnyby/fix-typo-in-mp_tree-docs-databse-databa-1429035855478

Without this fix, calling delete() on an inherited model would generate a queryset of descendants to be deleted, which only includes instances of that inherited model. Any descendants which are not of that specific subclass would thus be left behind as orphans.

--HG--
branch : cascading-delete-on-inherited-model-fix

Fixed ImportError in current Django `master` because `django.contrib.admin.util` became `django.contrib.admin.utils`

Change the docstrings of treebeard.mp_tree.MP_Node

Prevent the changelist causing an exception

Added get_annotated_list_qs and max_depth for get_annotated_list

Fix MoveNodeForm constructor

Further support for django 1.8 (and 1.7 without depreciationwarnings)

Limited tests (and hence support) to Python 2.7+/3.4+ and Django 1.6+. Updated tox.ini to the newer syntax