Stage 2 (of a currently-known 3) to properly close Issue #100.

Issue #100 [sounds](https://github.com/jdickey/new_poc/issues/100#issue-42742080) simple enough; implement a very straightforward use case.

This was *initially* envisioned as a two-stage process: Stage 1 modified the `Post` entity and `PostCCO` conversion class to support a new `:created_at` attribute and a `Post#add_to_blog` method (complementing and supporting `Post#publish`) that laid down the entity-level changes necessary to implement the new feature.

Stage 2, the branch addressed by this PR, added a (non-ORM) `:post_status` attribute to the `PostData` class and hooked up the new-post and edit-post forms with a new select-box control that is correctly initialised based on the status of the post (defaulting to `draft` for a new Post).

That *should* have been that, except that it turns out that a DSO reorganisation (as discussed in [these](https://github.com/jdickey/new_poc/issues/100#issuecomment-55787037) [comments](https://github.com/jdickey/new_poc/issues/100#issuecomment-55938026) on Issue #100 is urgently required, as the current logic marks *all* posts as `public` (which matches the status quo before beginning work on the issue itself).

Merging this branch back in and creating a new one for `issue-100-stage3` because, now that we've implemented the *letter* of Issue #100, it's time to finish making it meaningful.