docs(Scope): complete missing documentation for scope methods/properties

Closes #287
chore(test): fix analyze casting issues on ng_model_spec.dart
feat(forms): provide support for controls and state flags
feat(forms): add support for validation handling for multiple error types
feat(forms): provide support for parent form communication
feat(ngModel): provide support for custom validation handlers
feat(core): provide support to define the same selector on multiple directives
chore(docs): add docs for NgForm, NgModel and each of the NgModel validators