feat(validators): Add a pending state to AbstractControl

Add a pending state to AbstractControl and a function to set that state on
themselves and their parents. This will be used for both individual async
validators and when the imperitive mode is used. [Design Doc](https://docs.google.com/document/d/1EnJ3-_iFpVKFz1ifN1LkXSGQ7h3A72OQGry2g8eo7IA/edit?pli=1#heading=h.j53rt81eegm4)

modules/angular2/test/core/forms/model_spec.ts

@@ -479,6 +479,24 @@ export function main() {
         });
       });
 
+      describe("pending", () => {
+        var c: Control;
+        var a: ControlArray;
+
+        beforeEach(() => {
+          c = new Control('value');
+          a = new ControlArray([c]);
+        });
+
+        it("should be false after creating a control", () => { expect(a.pending).toEqual(false); });
+
+        it("should be false after changing the value of the control", () => {
+          c.markAsPending();
+
+          expect(a.pending).toEqual(true);
+        });
+      });
+
       describe("valueChanges", () => {
         var a: ControlArray;
         var c1, c2;