[Close #92] BLO classes removed.

`BLO::BlogDataBoundary` and `BLO::PostDataBoundary` classes and specs removed; client code updated. This was a long-festering sore suggesting that we didn't really have our architecture fully in sync.

Use of [message queues for decoupling](https://github.com/jdickey/new_poc/issues/95) and [standardising/unifying error messages](https://github.com/jdickey/new_poc/issues/94) have been decoupled from this as new issues in their own right.