Individual answers

* Add individual answers feature
* Update locales for main languages
* Migrate existing tests on new questions-answers model
* Allow to individualize answers: custom global answer title for question
* Use "Test Categories" in admin menu to differ with post categories

git-svn-id: https://plugins.svn.wordpress.org/wp-testing/trunk@1042822 b8457f37-d9ea-0310-8a92-e5e31aec5664