[Fix #12 #7] Completed "Adding Validation".

In the book, [Section 11](http://objectsonrails.com/#sec-11) adds model-
level validation for the `Blog::Post` model. This would actually be
quite straightforward were it not for a couple of quirks in the Mutation
error-reporting. That itself wouldn't be so bad, if I'd still had the
`#explain_errors` application helper method that appeared in previous
commits of the original `mutator` branch. Hilarity ensued while trying
to remember why that had been removed in the first place, apparently to
be replaced with the `#show_appwide_flashes` helper function (which
isn't what's needed here), and cope with what in the non-startup world
would have been a couple of sick days. So here we are.

Other items of note:
* Added the `active_attr` Gem, which gives us a (better?) way to make
  simple POROs that are compatible with ActiveModel for tasks such as
  forms;
* As mentioned above, `Blog::PostsController#create` now supports error
  handling (by recreating the failed post and adding errors to it);
* The `Blog::PostsController#new` method now calls a (new) Mutation,
  `NewPostForBlog`, removing the previous call to `@blog.new_post`;
* Previously-coded `ExplainErrors` module added back in to application
  helper module;
* `Blog::Post` model class includes `ActiveAttr::BasicModel`, which lets
  it "quack" like ActiveModel for the purpose of dealing with forms. It
  also adds an `errors` property and a `:title` field validation;
* The `NewPostForBlog` Mutation object is simply a shell around a call
  to the `#new_post` method on a `Blog` model instance, removing *any*
  knowledge of model internals from the controller itself;
* The 'new post' template has a call to the application-helper method
  `#explain_errors` added to it, to display error notifications as
  needed.

RSpec: 123 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 38 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 3
RuboCop local overrides in our source/spec files: 3