Xcode 5 Support

Hi,

I'm using your library on iOS 7 and recently updated to Xcode 5. I fixed some compiler warnings that now appear with these tools.

Could you please pull my changes and merge them into your project. All changes are fairly trivial.

Also: could you please create a tag and update the podspec at https://github.com/CocoaPods/Specs/tree/master/AQGridView. I'm happy to update the podspec, but I can't create the tag.

Thanks,
Claus