Added question type stubs

Sorry this took such a long time. It has been a crazy week. I'll be
working on properly implementing these classes tonight
Implemented TextResponseQuestion
Implemented PictureResponse and MultipleChoice questions

Note that the display as HTML logic is still missing.
Implemented FillBlankQuestion

Now these should just need HTML-rendering
Updated PictureResponseQuestion to allow for multiple answers

Todo: do the same with FillBlank, MC and Text