sharedManager renamed in sharedInstance to add compatibility to swift

[violabg]

if you use this library in a swift project, you can't access MHGallerySharedManager.sharedManager(). The compiler complains because it can't see sharedManager. I looked around and find a fix thanks to this article http://justinmiller.io/posts/2015/01/28/swift-objc-initializers/ So if you rename sharedManager in sharedInstance you are able to access the singleton instance like this: MHGallerySharedManager.sharedInstance().webPointForThumb