Merge pull request #1 from apache/optional-proper

Make PropEr an optional (test) dependency
tree: 5a0c8a1fa25bd9b4c50e8f5c47f80f131193e831
  1. c_src/
  2. src/
  3. test/
  4. .gitignore
  5. Makefile
  6. README
  7. rebar.config.script