Fix screen size

This pull request fixes an issue where if you set the screenWidth to be the same for multiple elements the screenshots for the second element aren't taken, instead you end up in an infinite loop talking screenshots with the name "screenWidthTest.test_four.undefined.png"

```
.webdrivercss('screenWidthTest', [
    {
        elem: '.test_three',
        name: 'test_three',
        screenWidth: [320,480,640,1024]
    },{
        elem: '.test_four',
        name: 'test_four',
        screenWidth: [320,480,640,1024]
   }
])
```

I have written a test but I could get any of the test to run so I'm not sure I have written it correctly.

Thanks,
David