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;

modules/angular2/test/public_api_spec.ts

@@ -47,7 +47,9 @@ var NG_API = [
   'AbstractControl.getError()',
   'AbstractControl.hasError()',
   'AbstractControl.markAsDirty()',
+  'AbstractControl.markAsPending()',
   'AbstractControl.markAsTouched()',
+  'AbstractControl.pending',
   'AbstractControl.pristine',
   'AbstractControl.setParent()',
   'AbstractControl.status',
@@ -277,7 +279,9 @@ var NG_API = [
   'Control.getError()',
   'Control.hasError()',
   'Control.markAsDirty()',
+  'Control.markAsPending()',
   'Control.markAsTouched()',
+  'Control.pending',
   'Control.pristine',
   'Control.registerOnChange()',
   'Control.setParent()',
@@ -304,7 +308,9 @@ var NG_API = [
   'ControlArray.insert()',
   'ControlArray.length',
   'ControlArray.markAsDirty()',
+  'ControlArray.markAsPending()',
   'ControlArray.markAsTouched()',
+  'ControlArray.pending',
   'ControlArray.pristine',
   'ControlArray.push()',
   'ControlArray.removeAt()',
@@ -345,7 +351,9 @@ var NG_API = [
   'ControlGroup.hasError()',
   'ControlGroup.include()',
   'ControlGroup.markAsDirty()',
+  'ControlGroup.markAsPending()',
   'ControlGroup.markAsTouched()',
+  'ControlGroup.pending',
   'ControlGroup.pristine',
   'ControlGroup.removeControl()',
   'ControlGroup.setParent()',