* Improved compatibility with UIActivityIndicator by providing a protocol compatible with both.
* Using a common protocol allows the user to make his implementation decoupled from this spinner or UIKit's default one.
* Improved compatibility with MBProgressHUD by implementing intrinsicContentSize and content size invalidation.
Signed-off-by: Goffredo Marocchi <goffredo.marocchi@gamesys.co.uk>