More Activities for the Share Button

This allows the user to add additional activities to the share button after initializing an FSImageViewerViewController.

Example (where reportActivity is a subclass of UIActivity): 
    
    FSImageViewerViewController *imageViewController = [[FSImageViewerViewController alloc] initWithImageSource:photoSource imageIndex:index];
    [imageViewController setApplicationActivities:@[reportActivity]];

Please use `_applicationActivities` (code convention)