Extended Exercises by worktime, difficulty and tags, added ProxyExercises as prework for recommendations

Tags can be added to exercises in the edit view. Tags can monitored under /tags.
Added the concept of ProxyExercises which are a collection of Exercises. They can be found under /proxy_exercises
Added Interventions as prework to show interventions later to the user.
Added exercise/[:id]/working_time to return the working time of the user in this exercise and the average working time of all users in this exercise

app/models/exercise_collection.rb

@@ -0,0 +1,5 @@
+class ExerciseCollection < ActiveRecord::Base
+
+  has_and_belongs_to_many :exercises
+
+end