Create test suite for PostgreSQL storage backend
PostgreSQL storage backend tests

Expand test suite for PostgreSQL storage backend and fix a bug that
expanded coverage detected.
Add more PostgreSQL storage backend tests
More tests for PostgreSQL
Add a [failing] test for PostgreSQL diff storage
Moved diff storing SQL to PostgreSQL store, changed schema, tidied up tests
Added a method to dump the Retcon database and added it to some of the tests
Added sorting to ensure consistency in test results
More content tests
Did some evil Document JSON serialisation magic to allow flat values to work when serialised. All pretty nasty tbh but the tests pass
Added contents check for all PostgreSQL tests
Delete unused module
Merge pull request #31 from anchor/wip-pgstore-tests

Add test coverage for the PostgreSQL storage backend