Fix namespaces in AutoPatternsMixin

+ remove guess_url_namespace attribute and args)
+ pass namespaces to get_url_patterns, until get_url_name where it can be
passed to reverse)
+ various code cleanup in get_url_patterns* functions
custom URL prefix with get_url_prefix (and tests)

(get_url_prefix still missing, see next commits)
Test reordering in tests/test_model_mixins.py (and add tests for
get_url_prefix and get_url_namespaces)
Cleanup auto_patterns_for_app (and add possibility to exclude models)

Add supple_join (beeeeeeeeeeerrrrkkk)
Code reorganization in models.mixins, (and major changes not committed
on time, sadly)