Add ability to customize DBCameraSegueViewController using a block

This functionality was developed in response to #37. I use this functionality to require users to crop to certain aspect ratio.

Usage:
```
DBCameraViewController *cameraController = [DBCameraViewController initWithDelegate:self];
[cameraController setCameraSegueConfigureBlock:^(DBCameraSegueViewController *segue) {
    segue.cropMode = YES;
    segue.cropRect = CGRectMake(0, 0, 200, 400);
}];
```

This change also automatically centers cropping rectangles on the screen for easier usage.

I have noticed that dragging and dropping while cropping sometimes causes the dragged image to snap back to previous position. This is not caused by this change, but I am not sure how to fix it.