connect assessments engine (selects next exercise) to frontend via new API routes
change assessments API route as suggested in review K100348
Merge branch 'master' into assessment
moved setting of assementMode into prepareSite which is called after Exercises.init()

Summary: assessmentMode wasn't being set correctly because Exercise.init() had not been called yet

Test Plan: Load /labs/assessments/fractions page and get a problem wrong and make sure that it doesn't say try again

Reviewers: chris, jace

CC: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D1074
changed the api call to use user_assessment_key rather than the assessment slug
Merge branch 'assessment'