Make maxRating is more robust & update class comment

- `maxRating(String value)` is now more robust (will handle situations
where `value.isEmpty` (which happens when `max-rating=“{{…}}”` as
suggested in the tutorial.
- Updated class comment: we are not using the `@NgComponent` `map`
field anymore.
[chap3] maxRating() made more robust and component class comment updated

- `maxRating(value)` made more robust; it can now handle when
`value.isEmpty()`, which can happen when `max-rating=“{{…}}”` using the
mustache directive.
- Updated class comment: we do not use the `@NgComponent` `map` field
anymore.
Propagation of chap 3 updates to subsequent chapters
Merge branch 'master' into patch-chap3-comp

Conflicts:
	Chapter_07/lib/rating/rating_component.dart - resolved.
Propagating capitalization fix from Chapter 2 to Chap 3 to 6

Angular Component -> Angular component.
Merge pull request #69 from chalin/patch-chap3-comp

[chap3] maxRating() made more robust and component class comment updated