Add tests for abbr_role. Refs #949
Fix bug with custom reST :abbr: role. Fixes #949

This fixes things so that newlines are allowed within the explanation of
an :abbr: role in reST mark-up.
Merge pull request #1507 from dairiki/bug.rst-newlines-in-abbr

Fix for #949 bug with custom reST :abbr: role