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/test/public_api_spec.ts

@@ -427,6 +427,8 @@ var NG_COMMON = [
   'ObservableListDiffFactory.create():dart',
   'ObservableListDiffFactory.supports():dart',
   'ObservableListDiffFactory:dart',
+  'PatternValidator',
+  'PatternValidator.validate()',
   'PercentPipe',
   'PercentPipe.transform()',
   'RequiredValidator',
@@ -452,6 +454,7 @@ var NG_COMMON = [
   'Validators#maxLength()',
   'Validators#minLength()',
   'Validators#nullValidator()',
+  'Validators#pattern()',
   'Validators#required()',
   'RadioButtonState',
   'RadioButtonState.checked',