answerOptions images and radio buttons

[smcnally]

• answer option images should have their own div and fill 100% of it
• radio selects should overlap the upper-left of the answer option images
• add a white bg to the radio selects to distinguish it from the image

[smcnally]

Using this execution as example: https://meanbusiness.com/shepherd/cannabisfeminist/

Along these lines is better:

• Image is centered; some consistent padding all around
• radio and answer label float on the image

Too many of these elements are currently handled by the same radioCustomLabel style.

[rogertheriault]

Sorted in https://github.com/smcnally/office_quiz/pull/4