made AssignmetPlan act_as_taggable; updated AP form to allow addition of tags to assigned APs
fixed AP form window height; fixed combining of multiple labels into one
fixed migration confusion caused by old/new versions of AssignmentPlan#exercise_tags methods
removed AssignmentPlan#exercise_tag methods and usages
added :new_exercise_tags accessor
Merge pull request #254 from kevinburleigh75/add_tags_to_assignment_plans

Allows labels to be added to assigned AssignmentPlans