Added PlaceholderBlogEntriesForDevelopment service object.

This simply wraps the ["Placeholder blog entries"](http://objectsonrails.com/#ID-6c9ee215-55f5-477d-be6c-0e3de4f3b663) step in the book.

Also added new configuration item to Rubocop config file, to exclude `placeholder_blog_entries_for_development_spec.rb` from the [TrivialAccessors](https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/cop/style/trivial_accessors.rb) cop.

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