fix exception when call mocha HTML reporter

GruntReporter will rise an exception when initialize:

```js
  mochaInstance.reporters.HTML.call(this, runner);
```

because `HTML` reporter will listen some event which need `suiteURL`
method:

```js
  function HTML(runner, root) {
    // ...
    runner.on('suite', function(suite){
      // ...
      var url = self.suiteURL(suite); // will rise an exception
      // ...
    });
    // ...
  }
```

So make GruntReporter inherit HTML reporter to resolve this issue.