Adding the `Blog::Blog.entries` attribute.

Following along with *Objects on Rails*' [Adding blog entries](http://objectsonrails.com/#ID-d180a5f4-e2e9-4506-8ea0-77445e39a512) page, which was *his* first mention of specs. Yes, they slowed us down previously — but now we don't leave any black boxes (or holes) behind.

RSpec: 26 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 13 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
Revisiting Issue #1.

The `PublicBlogLIsting` service object no longer creates a `Blog::Blog` model instance; it takes one as its (only) parameter and uses it to build the return value on success. Since the app presently has no authentication or authorisation on which to filter entries (that don't yet exist themselves, but this class doesn't know that), the `blog` parameter itself is returned from `execute`.

In other news:
* Specs duplicating those of the `Blog::Blog` model have been removed from the spec for `PublicBlogListing`.
* One new entry added to the project-global RuboCop configuration file, overriding default line length limit of 79 (for 80).
* README updated with new "Interface Segregation" criterion. This version of the README will also be added to the master branch, and the `interactors_start_here` tag moved to the ensuing commit.