pluggable permissions
Merge branch 'master' into pluggable_authorization
fix several bugs (all unittests should pass now)
add documentation
Merge branch 'master' into pluggable_authorization

Conflicts:
	pybb/defaults.py
update permissions patch for LatestTopicsView
remove unnecessary double instantiation of queryset
Merge branch 'master' into pluggable_authorization
Merge branch 'master' into pluggable_authorization
Merge branch 'master' into pluggable_authorization
moved all permission-related checks to permission handler
added unittest for custom permission handler
remove unused functions
don't allow subscribing to topics which one is not allowed to see (potential information leak)
Merge branch 'master' into pluggable_authorization
fixed #42 prevent duplicate read trackers by set unique_together meta attribute

existing duplicates removed by data migration
fixed tests for custom permission handler, little code cleanup
Merge branch 'master' into pluggable_authorization_orig

Conflicts:
	pybb/views.py
fixed merge error
Merge pull request #55 from hovel/pluggable_authorization

Pluggable authorization, see #47
trying to get forum/topic read tracker after inserting duplicate row
Merge branch 'pluggable_authorization_orig'