Merge pull request #229 from APSL/master

Modify javascript to also consider the iframes.
Fix exclude for nullable field Manager Rewriting (#231).
Use _default_manager instead of objects; patch _default_manager as well.
PEP8 fix.
Capitalize group_fieldsets titles (close #234).
Fix tests to reflect changes in group fieldset capitalization (ref #234).
Add more control over required languages (close #143).
Fix new code to be Python 3 compatible.
Fix .values() with no fields specified (close #236).
Fix test: there already was a method called test_values.... (ref #263).
Fix values_list without fields; refactor tests (ref #263).
Repair tests: don't use absolute pk values.
Small docs update.
Satisfy new flake8 version.
Merge remote-tracking branch 'upstream/master'