Image swapping with ng-click

 In the phone detail view, clicking on a thumbnail image, changes the
 main phone image to be the large version of the thumbnail image.

 - Define mainImageUrl model variable in the PhoneDetailCtrl and set its
   default value
 - Create setImage controller method to change mainImageUrl
 - Register ngClick handler for thumb images to use setImage controller
   method
 - Add end-to-end tests for this feature
 - Add CSS to change the mouse cursor when user points at thumnail images