Feature/9 better logging than full evdata

This would close #9, although I suspect we may potentially want a better form than json? and that helper func may need to move?
I think logging json here is fine as that makes parsing in logcabin easier. All places where we log raw python I've ended up regretting it. I would recommend you log "%s" though and add an `.encode('utf-8')` on the `str` because Twisted gets very unhappy with logging anything non-bytestring