added methods and routes for getting an exercises history, and the sum of a workouts musclearrays -- incl a modifier

data/schemas/workout.js

@@ -8,7 +8,8 @@ var Split = new Schema({
 
 
 var Element = new Schema({
-  exerciseID: String,
+  exerciseID: { type: Schema.ObjectId, ref: 'Exercise'},
+  modifier: {type: Number},
   splits: [Split]
 });