v.1.5 Add "data" handling.

Allow for configurable (default 1024) bytes of data to be sent to
directly connected clients.

@kitcambridge, @bbangert r?
Needs a matching [`LoadConfigForSection` call](https://github.com/jrconlin/pushgo/blob/feature/data2/src/github.com/mozilla-services/pushgo/simplepush/config.go#L335-341) in `config.go`, too.