Add `hasTasks` computed property for Projects

[hchood]

Need hasTasks property to reference on the project show page so I can display a message "No tasks added yet" instead of the "Tasks" header and <ul>.

Can I put that on the model? Examples I've seen put it on the controller. Need to better understand what can go on the model vs. controller.

http://stackoverflow.com/questions/25436210/ember-js-length-of-array-of-related-objects
http://emberjs.com/guides/object-model/computed-properties-and-aggregate-data/