Bug fixes and a new delegate method

Hi,

I'm using your framework in one of my projects. Great work! I think its one of the best photo browser frameworks right now.

I had to make some changes to fully fulfill my need. If you want you can pull them into your repository and update the cocoapods version.

So here are my changes. 

1. There was a bug with the custom button view. It was always set to origin.y = 0 but it should be on the button of the screen.

2. I added an additional delegate method to get notified if the currentIndex changes.

3. There was a bug where remote images couldn't be zoom. This occurred if the image was already in the image cache.