Post model validates presence of :title attribute.

We've been including *bits and pieces* of ActiveModel, not the whole
thing. This is where Avdi quits stubbing anything in ActiveModel and
includes the real thing (including or extending three AM modules in this
model in the process) as a precursor to the imminent subclassing of
ActiveRecord::Base. We're going to attempt a different tack with the use
of a (mostly) workalike library, Chris Griego's `active_attr`.

RSpec: 90 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 27 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5
"Member" can create a valid post by entering title+body.

Of course we don't have authentication yet, so "member" is presumptive,
but still. Default happy path spec covered.

RSpec: 91 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 28 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5
Post model now validates by checking for non-blank :title.

New feature spec is *meant* to prove that this works. However, we don't
yet have reported errors being added to the views. Coming right up…

RSpec: 94 examples, 0 failures, 1 pending
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 29 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 5