Merged.

Many thanks! Diff looks good to merge.

I renamed the functions and the code now uses ostringstream instead of
stringstream in the places you mentioned.

> I suggest to do the renaming separately to keep the diff small.

I don't think it would be hard to review, and if it can be easily avoided, an
issue should not leave the default branch in a broken state. I think a
combination of "format_paper_reference" and "format_journal_article_reference"
is a (mildly) broken interface because it's an inconsistent level of
abstraction. (A journal article reference is a paper reference.)

Regarding the name, "journal article" is a better word in isolation, but
"journal paper" is sufficiently common, and it's good to have parallel names for
parallel functions. Another option would be "format_conference_reference" and
"format_journal_reference". But I'm also fine with your names.

I suggest to do the renaming separately to keep the diff small.

Here is a pull request:
https://bitbucket.org/jendrikseipp/downward/pull-requests/130

I called the function "format_journal_article_reference" since "journal article"
yields more results than "journal paper" on Google but am indifferent regarding
the name.

Title sounds good! I would suggest renaming "format_paper_reference" to
"format_conference_paper_reference" at the same time for clarity.