Re-export Open

https://github.com/lib/pq/pull/161#commitcomment-4297782 shows I was
mistaken in assuming people don't use this function directly.
Make option parsing more conformant to libpq behaviour.

Fixes #162
go fmt
Merge pull request #166 from paulhammond/gofmt

go fmt
Note that users no longer need to call ParseURL.
Add docs for getting inserted row ids and parameter markers