Add command for saving screenshots in the camera roll

Hi,

@drodriguez and I've added a command for populating with photos the camera roll of an iOS device/simulator. We needed this in order to test our app properly, because our acceptance tests reset the simulator data before launching the app, assuring a clean environment for each scenario, but losing any photos we may have in the camera roll.

Therefore, we thought the cleanest way of putting images in the camera roll was adding a customised step for Frank so that each test could have different number of photos in the camera roll for different purposes.

Usage: just add the following step to an scenario
```cucumber
Given the device saves 7 screenshots in the camera roll
```

We think this command may be useful for more people, so if you like it feel free to merge it. If you think it needs some changes before merging, just tell me and I'll do my best :)

Thanks in advance!