REFACTORING: simplified SHKSharer.h by creating SHKSharer_protected.h, with methods intended to be only used by subclasses.
NEW FEATURE: introduced SHKSendProgressNotification. It is sent during each incremental send progress on large files. Actual progress value can be read via [notification.object progress]
NEW FEATURE: added SHKActivityIndicatorSubclass configuration method. Use this, if you wish different activity indicator.