Proper PostgreSQL support

[evert]

This is a new version of PR #521. I started from scratch but used #521 for inspiration.

Tackling PostGres one unittest at a time.

Fixes #804

[evert]

The work for this is functionally done. Everything works, everything is tested locally. We will only support postgres 9.5.

Two steps are still left:

1. I need to tell travis somehow to install postgres 9.5
2. I want the unit test suite to automatically skip tests if we're connected to a postgres server that is older than 9.5.

After that, this can be merged.

[evert]

Looks like there's currently not a sane way to enable postgres 9.5 in travis. So I'm leaving those tests skipped for now. All tests pass locally though, so postgres support is completed!