API cleanup

[codemasher]

Ok so that's the last change proposal for now, for a future v3.x.

• It removes the abstract classes and traits - each test is now self-contained in order to ease extending if necessary.
• Further, all properties and methods have type hints added.
• Some superfluous assertions (such as assertInstanceOf for objects received from the factory methods with return types) and custom assert methods have been removed.
• Properties that hold instances of the factory methods are now named after the factory ($requestFactory vs. $factory).
• I made 3 separate commits so that the change history remains traceable.

I've spent more time for this now than I planned - I'm back on qrcode stuff if you need me.