Support AppConfigs in INSTALLED_APPS (close #252).

Thanks Warnar Boekkooi for initial commit.
Add docs section about debug toolbar.
Fix MultilingualManager MRO (close #204).
Rewrite field names in select_related; fix deffered models registry lookup (close #248).
Rewrite spanned queries on all levels for defer/only (close #248).

Also:
- optimize fields to related model access
- more elegant way to access DefferedModel real class
- fix select_related rewriting and add some code to contemplate possible solutions
- update docs about select_related
- add 2 test for implemented features
Fix reduce usage under Python3.
Fixed translation fields in migrations don't include language code (close #253).
Fixed unicode call in deconstruct to maintain Python3 compatibility.
Added Python 3.4 and changed Django to 1.7rc1 (ref #254).
Used six compatibility layer (ref #253).
Indicated Python 3.4 support (close #254).
Fix six import (ref #253).