Added support for results of share dialog presenting

Several changes in VKShareDialogController. Added support to receive with handler results of user actions.

Works in similar way as SLComposeViewController in Social.Framework.
- You can add to VKShareDialogController block property that will be called after "done" or "close" action
- Type of block is VKShareDialogControllerCompletionHandler
- Block receives only one simple parameter of type VKShareDialogControllerResult