[tutorial] main page: add ng-model, and @ to annotations

Partly addresses #23.
Merge pull request #25 from chalin/patch-tut-anno-naming-0220-1

[tutorial] main page: add ng-model, and @ to annotations
fix a typo
Merge pull request #26 from vicb/patch-5

fix a typo
Merge branch 'master' of https://github.com/angular/angulardart.org

Conflicts:
	app/tutorial/index.html