v.1.5 Add "data" handling.

Allow for configurable (default 1024) bytes of data to be sent to
directly connected clients.

@kitcambridge, @bbangert r?
FWIW, the Ruby [APNS client library](https://rubygems.org/gems/gs-apns) handily provides truncation...though I'd prefer if the payload were opaque to us. App servers can truncate payloads as needed, but I think that should be their decision.