Added close.js to commands

This allows to close the browser window.

Sometimes browsers might not be closed by their driver if the session is ended. I can reproduce this problem with Chrome on Windows. Closing the window before ending the session cleans evenything up.

I'm doing this:

```js
client.close(function () {
  client.end(function () { /* ... */ });
});
```

I had difficulties with the test setup. Firefox had lots of failures in the build and running the tests in the cloud was terribly slow. I gave up on getting the test env to work properly, therefore no test. Sorry.