Question paper generator

QuestionPaper model modified for generating random questions.
Added language field to Question model.
Removed tags from Quiz model.
Template and javascript for Add and Edit questions modified.
Added test cases for models.
Usually, each model is tested in a different class.  For example `TestUser`, `TestQuestionPaper`, `TestQuiz` etc.  In this case you have put all of them in one big class.  Usually each model is also in a separate file but in this case perhaps you can put them in one file.