Handle rotation

While using GBInfiniteScrollView in a universal app, I ran into problems handling rotation. It seemed that pages created in one orientation didn't properly resize once the app had rotated to a different orientation.

In order to fix this problem, I made the following changes:

- GBInfiniteScrollViewPage specifies `autoresizingMasks` for its `contentView`, `imageView`, and `textLabel`
- GBInfiniteScrollView ensures that page frame sizes are set correctly during placement (`placePage:atPointX:`, `placePage:atPointY:`, etc)

The host application still needs to inform GBInfiniteScrollView that the rotation has occurred (ex: by calling `resetLayout` at the appropriate time). In my iOS 8 app, I use:

```
- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
{
    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context) {
                                                [infiniteScrollView resetLayout];
                                            }
                                 completion:nil];
}
```

Which seems to give the expected results.