#359 - no longer requiring Now when raising events

Fixes #359.

Includes functionality that had been discussed as an add-on to #30 (and will enable changes for that issue).
I think this should be named `TryRemoveArgumentProviderFor` to convey removal semantics. Otherwise, I'd be surprised that I couldn't `Get` repeatedly.

Perhaps it could even be `TryGetAndRemoveArgumentProviderFor`, if we feel that the `out` parameter doesn't convey the `Get` semantics clearly enough.