Support an initial sub-set of the extended query message flow

[KayEss]

Version 0.1 brought in support for the simple query mechanism. This is actually pretty useful on its own as it already supports essentially anything you can do with Postgres. It does have some drawbacks though, most importantly (for our use case) is that it requires everything to be turned into SQL statements, and this opens issues about things like SQL injection attacks.

Better is to use the extended query message exchange which allows the data to be embedded directly in network packets in a way that doesn't require textual escaping. And Fostgres uses prepared statements anyway, so we need it.

This is a big feature, so to start with I just want to see the basics working so we can use prepared statements for data insertion. It's a bit parochial I know, but at that point this library covers our use cases and I'd be relatively happy to call it "1.0" :)