feat(command): add "maximize window" command

Add a new command to maximize the current browser window.

```
browser
  .url('http://nightwatchjs.org')
  .maximizeWindow(function(){
    console.log('window is now maximized');
  })
  .end();
```

@beatfactor
This is just quickly hacked together. I'll add some tests later.
Would be great if you could quickly scan this just to make sure i'm on the right path.. 

regards
~david

can you also validate the callback since it's optional?