Modified question_group_methods and added tests

This fixes #438.

I am not very familiar with FactoryGirl and how to associate questions to question_group, so I modified the translation method in question_group to simply return the text and help_text hash when the question_group doesn't have any questions.

Commit message: Changed a_text(a) to a.text_for in show.html.haml, replaced text_for in question_group_methods with same method in answer_methods, which takes three parameters, added tests (mostly copying from other tests), and modified translation method to return hash when question_group doesn't have questions. Also copied
split, imaged, and part_of_group? methods.