Added author_name field to post_data table.

RSpec: 401 examples, 0 failures
RuboCop: 111 files inspected, no offenses detected
Added Post author name to models and interactors.

Big-bang commit showing what it takes to get back to a green bar. Only
work remaining before merging branch back to master is getting the blog
index view updated to show the articles' author names.
Added PostDataDecorator#build_byline method.

Adds support class `BylineBuilder`, which takes the (possibly
decorated) PostData model as a parameter to `#initialize`.
`BylineBuilder#to_html` returns the markup for the article's byline,
for use in the blog-entry view.

RSpec: 404 examples, 0 failures
RuboCop: 112 files inspected, no offenses detected
Merge pull request #51 from jdickey/issue-36

Fixes #36.