all tasks are generic and have steps

also, exercise step has a content JSON key
remove exercise.parts
remove exercise.parts from styles
render question.stimulus
remove question.answer.value
rename exercise.background to exercise.stimulus
Merge pull request #5 from openstax/update-exercise-json

Update exercise json
Merge pull request #4 from openstax/all-tasks-have-steps

all tasks are generic and have steps