Support unmounting in ReactShallowRenderer

Reviewers: @sebmarkbage, @zpao

Test Plan:

```
$ npm test ReactTestUtils
```

src/test/ReactTestUtils.js

@@ -334,7 +334,7 @@ var NoopInternalComponent = function(element) {
   this._currentElement = element === null || element === false ?
     ReactEmptyComponent.emptyElement :
     element;
-}
+};
 
 NoopInternalComponent.prototype = {
 
@@ -349,7 +349,6 @@ NoopInternalComponent.prototype = {
   },
 
   unmountComponent: function() {
-
   }
 
 };
@@ -374,6 +373,12 @@ ReactShallowRenderer.prototype.render = function(element, context) {
   ReactUpdates.ReactReconcileTransaction.release(transaction);
 };
 
+ReactShallowRenderer.prototype.unmount = function() {
+  if (this._instance) {
+    this._instance.unmountComponent();
+  }
+};
+
 ReactShallowRenderer.prototype._render = function(element, transaction, context) {
   if (!this._instance) {
     var rootID = ReactInstanceHandles.createReactRootID();

src/test/__tests__/ReactTestUtils-test.js

@@ -57,6 +57,23 @@ describe('ReactTestUtils', function() {
     ]);
   });
 
+  it('should have shallow unmounting', function() {
+    var componentWillUnmount = mocks.getMockFunction();
+
+    var SomeComponent = React.createClass({
+      render: function() {
+        return <div />;
+      },
+      componentWillUnmount
+    });
+
+    var shallowRenderer = ReactTestUtils.createRenderer();
+    shallowRenderer.render(<SomeComponent />, {});
+    shallowRenderer.unmount();
+
+    expect(componentWillUnmount).toBeCalled();
+  });
+
   it('can shallow render to null', function() {
     var SomeComponent = React.createClass({
       render: function() {