Blog entries are sorted and limited.

Accomplishes the goal as described in Section 10 of the book, BUT:
* Sorting and limiting are presentational, and therefore should not be
  in the model;
* Fixed count is ugly, for precisely the reason described at line 63 of
  the current blog model spec; once an entry has slipped "off the end",
  it's unretrievable.

Coming up: going off-road just a tiny bit, to introduce a #fetch_entries
decorator method to address these concerns.

RSpec: 88 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