#65 adding bootstrap.css to exercise instructions

for issue #65 "Add Bootstrap css to exercise instructions"
Hm. You can't use dot or brackets notation for attributes-- you could only use that for attaching properties to the model object that weren't considered attributes.
Maybe "If you try to access them directly as object properties via dot/bracket notation, they'll be undefined."?
I feel like it's better at this point to point out what errors they will run into if they forget to use get/set, instead of introduce the possibility that they can actually avoid get/set for legitimate reasons.