[ShareKit/remind101-master] Added method to instantiate actionsheet with sharers. Changed sharers property to readonly since it doesn't act as a read-write property.

Originally, the .sharers property was (retain), and thus read-write and atomic. This doesn't actually match how SHKActionSheet functions however. Setting the sharers property put any object instantiated by this class in an inconsistent state.

Instead, I changed .sharers to readonly (and still atomic) and added the method
'+ (SHKActionSheet *)actionSheetForItem:(SHKItem *)item withSharers:(NSArray *)sharers'
which is now the default allocator amongst the two provided in the API. Providing a nil 'sharers' parameter will default sharers to the 'favoriteSharersForItem:'