make startLoadingExercise available to the outside via Khan so that assessments can access the promise of the exercise html file having been loaded

Test Plan:
go to an assessment
add breakpoint in makeProblem
make sure that the old problem isn't shown yet

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2089
Merge branch 'arcpatch-D2089'