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