refactor(forms): use multibindings instead of query to get a list of validators

BREAKING CHANGE

Before:

@Directive({selector: '[credit-card]', bindings: [new Binding(NgValidator, {toAlias: forwardRef(() => CreditCardValidator)})]})
class CreditCardValidator {
  get validator() { return CreditCardValidator.validate; }
  static validate(c): StringMap<string, boolean> {...}
}

After:

function creditCardValidator(c): StringMap<string, boolean> {...}
@Directive({selector: '[credit-card]', bindings: [new Binding(NG_VALIDATORS, {toValue: creditCardValidator, multi: true})]})
class CreditCardValidator {}

modules/angular2/test/forms/validators_spec.ts

@@ -33,6 +33,9 @@ export function main() {
     });
 
     describe("compose", () => {
+      it("should return a null validator when given null",
+         () => { expect(Validators.compose(null)).toBe(Validators.nullValidator); });
+
       it("should collect errors from all the validators", () => {
         var c = Validators.compose([validator("a", true), validator("b", true)]);
         expect(c(new Control(""))).toEqual({"a": true, "b": true});