Add a callback that handles the changes in the state of a sticky header such as "onPinned"

[fujidaiti]

This PR fixes #48 and #70 by adding a callback to be called when a sticky header is pinned or unpinned, etc. It also works with reversed scrollable widgets.

• Added SliverStickyHeaderActivity class that represents an event such as "onPinned"
• Added the event dispatching logic to RenderSliverStickyHeader
• Added callback parameters to the constructors of SliverStickyHeader
• Added a new example to example/lib/main.dart

I hope you find it useful 😀