test(directives) Added more tests for contenteditable directive. And c...

Hi,
After some time beating my head trying to make all testes pass. I find that element.isContentEditable do not work.
But that is not a problem in the browser, only in the test. I don't know way, so I open the pull request as a review. I'm 99.9% certain that all tests are correct and pass in real environment.

The last test is to see the problem:

```dart
    /**
     * If this test do not pass, several contentEditable test will fail
     */
    it("should the context contenteditable be truthy", inject((){
      _.compile("<p contenteditable probe='p'></p>");
      _.rootScope.$digest();
      Element element = _.rootScope.p.element;
      expect(element.isContentEditable).toBeTruthy();
    }));
```

Is this a problem in the _.compile method? Why isContentEditable not true?