Placeholder blog entries; useful dummy content for *development*.

This simply wraps the ["Placeholder blog
entries"](http://objectsonrails.com/#sec-5_1) step in the book.

It also marks the first inline RuboCop override in our code; we disable
the `TrivialAccessors` cop around the method DummyPost#published? in the
spec file; the method *is* a trivial accessor for the `@was_published`
instance variable. However, the idiomatic Ruby accessor name for a
Boolean value `some_flag` would be `some_flag?`, which is a legal name
for a function, but *not* a valid variable name. Pffft.

RSpec: 27 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 15 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 4
RuboCop inline overrides in our code/specs: 1