Use ng-swtich for the conditional display of Loading message

- Only show “Recipe Details” heading when there is a recipe (like in
the other chapters).
- Use opportunity to introduce `ng-switch` for the conditional display
of the "Loading ..." message.
- Propagate changes to chap. 6 -7 .
Merge pull request #90 from chalin/patch-0226-chap5-ng-switch

Use ng-swtich for the conditional display of Loading message