Implementing workout and exercise lists

Even before we start implementing the Workout and Exercise list pages, we need a data store for exercise and workout data. The current plan is to have an in-memory data store and expose it using an Angular service. In Chapter 3, Supporting Server Data Persistence, where we talk about server interaction, we will move this data to a server store for long-term persistence. For now, the in-memory store will suffice. Let's add the store implementation.