replays and upcasting

It is now possible to call Storage.replay_events in order to run all the handlers again.

There were some breaking changes:

• handlers now require another argument: replaying_events. It would normally be a False and True only during events replay process. This is to make sure that projections updates are always run while all the other side effects can be omitted
• It is now required to pass aggregate_type to Storage.get_events function. We've split Event.name into Event.aggregate_type and Event.name. Example: {name: "User.Registered"} would become {aggregate_type: "User", name: "Registered"}

This release introduces main missing features. Next release would focus on

• nicer API
• events schema definition and validation
• events monitoring and/or better querying