Code formatting: space associations better
Associate postmeta with posts

Previously the relationship was in one direction only (i.e. posts had
many postmeta, but postmeta did not have one post).
Unit test cleanup

DRY up some tests, make sure created objects are destroyed
Bugfix for post parent-child relationship

- Ensure we link post_parent and ID properly
- Also add Post#parent, so we can easily reference the parent post of
  a child