Default to post author if post editor is missing

Fixes #100
Use customizable deleted-user structure
Enforce string value for display_name
Add missing ')'

Staged only part of a change -- stupid mistake.
Deleted-user variable is not risky
Merge branch 'master' into issue-100
Merge branch 'master' into issue-100
Merge branch 'master' into issue-100

Conflicts:
	sx-question-mode.el
Reimplement deleted-user
Merge pull request #101 from vermiculus/issue-100

Default to post author if post editor is missing