[tutorial] Chap 5: introduce ng-switch and new exercise

- Use opportunity to introduce ng-switch for the conditional display of
"Loading ..." message.
- Summary: mention that Chap 5 covers JSON, ng-switch.
- Chap. 5: add extra exercise (building upon the first two).
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Merge pull request #36 from chalin/patch-0226-chap5-varia

[tutorial] Chap 5: introduce ng-switch and new exercise