create onSave and onLoad events for overloading

instead of overloading save and load and then calling the parent, create static methods called onSave($params = []) and onLoad($params = []) that are to be overloaded. this will allow the use of both static and object context without problems.

Triaged for Tipsy 2.0 (#57)

Still open. No onSave / onLoad hooks; override save / load (or the __ magic methods) directly. Related to #17 / #25.