feat(forms): add support for adding async validators via template

Example:

@Directive({
  selector: '[uniq-login-validator]',
  providers: [provide(NG_ASYNC_VALIDATORS, {useExisting: UniqLoginValidator, multi: true})]
})
class UniqLoginValidator implements Validator {
  validate(c) { return someFunctionReturningPromiseOrObservable(); }
}

modules/angular2/test/public_api_spec.ts

@@ -819,6 +819,7 @@ var NG_ALL = [
   'LowerCasePipe',
   'LowerCasePipe.transform()',
   'NG_VALIDATORS',
+  'NG_ASYNC_VALIDATORS',
   'NgClass',
   'NgClass.doCheck()',
   'NgClass.initialClasses=',
@@ -837,6 +838,7 @@ var NG_ALL = [
   'NgControl.untouched',
   'NgControl.valid',
   'NgControl.validator',
+  'NgControl.asyncValidator',
   'NgControl.value',
   'NgControl.valueAccessor',
   'NgControl.valueAccessor=',
@@ -857,6 +859,7 @@ var NG_ALL = [
   'NgControlGroup.valid',
   'NgControlGroup.value',
   'NgControlGroup.validator',
+  'NgControlGroup.asyncValidator',
   'NgControlStatus',
   'NgControlStatus.ngClassDirty',
   'NgControlStatus.ngClassInvalid',
@@ -884,6 +887,7 @@ var NG_ALL = [
   'NgControlName.update=',
   'NgControlName.valid',
   'NgControlName.validator',
+  'NgControlName.asyncValidator',
   'NgControlName.value',
   'NgControlName.valueAccessor',
   'NgControlName.valueAccessor=',
@@ -941,6 +945,7 @@ var NG_ALL = [
   'NgFormControl.update=',
   'NgFormControl.valid',
   'NgFormControl.validator',
+  'NgFormControl.asyncValidator',
   'NgFormControl.value',
   'NgFormControl.valueAccessor',
   'NgFormControl.valueAccessor=',
@@ -996,6 +1001,7 @@ var NG_ALL = [
   'NgModel.update=',
   'NgModel.valid',
   'NgModel.validator',
+  'NgModel.asyncValidator',
   'NgModel.value',
   'NgModel.valueAccessor',
   'NgModel.valueAccessor=',
@@ -1223,6 +1229,7 @@ var NG_ALL = [
   'UrlResolver',
   'UrlResolver.resolve()',
   'Validators#compose()',
+  'Validators#composeAsync()',
   'Validators#nullValidator()',
   'Validators#required()',
   'Validators#minLength()',