Added notice about Model's multi-inheritance

When using multi-inheritance, MPTTModel should usually be the first class to be inherited from.
Merge pull request #222 from pstch/patch-1

Added notice about Model's multi-inheritance
Added parent caching to cache_tree_children filter (test included)
Removed QuerySet dependency from updated cache_tree_children
Minor optimization
Merge pull request #226 from orokusaki/master

Updated `cache_tree_children` to include parent caching. thanks @orokusaki
add failing test case for #232
Merge branch 'master' of github.com:django-mptt/django-mptt

Conflicts:
	tests/myapp/tests.py
Fix tests on django 1.2
Fix tests on django 1.2
Start making django-mptt compatible with PY3

Requires Django 1.5.