feat(forms/validators): pattern validator

Adding static pattern validation method to Validators

Adding a directive for the pattern validator

Applying clang-format rules to modified files

Updating public api spec for new pattern validator

Adding pattern validator to public api guard tool

For #5411

Closes #5561

modules/angular2/src/common/forms.ts

@@ -37,6 +37,7 @@ export {
   RequiredValidator,
   MinLengthValidator,
   MaxLengthValidator,
+  PatternValidator,
   Validator
 } from './forms/directives/validators';
 export {FormBuilder} from './forms/form_builder';