Consider hiding fields in Event

[erickt]

As part of developing #307, I realized it might be worth hiding the public fields in Event as part of 5.0.0. This would allow the underlying object to be extended with new attributes without breaking existing clients. I have a proof-of-concept of this change in https://github.com/erickt/notify/tree/hide-fields that's built upon #307 that I can push up after we address #306.

[0xpr03]

I've got no real stakes in this, we've had no problems with them being public so far. Also your current implementation adds an additional .clone - though its probably irrelevant. So I'm ok with merging it.