Added option to include a image to question text

[AntonioLagoD]

Hello.
I created this pull request for enhance question.py so it is possible to add an image to the question text. Image is optional so you can use:

q2 = MultipleChoiceQuestion(name = "Celsius Fahrenheit Equivalency IMG",
                            question_text = "At what temperature value do the Fahrenheit and Celsius scales have the same numerical value?",
                            image_name="image1.jpg",
                            default_mark = 1,
                            correct_feedback = "Correct!",
                            shuffle=True)

or

q2 = MultipleChoiceQuestion(name = "Celsius Fahrenheit Equivalency IMG",
                            question_text = "At what temperature value do the Fahrenheit and Celsius scales have the same numerical value?",
                            default_mark = 1,
                            correct_feedback = "Correct!",
                            shuffle=True)

I created a second commit just to create the appropiate tag to shuffle the answers in a question.

It would be great if you accept the pull request and publish the new package in PYPI.
Cheers.