Fallback TEMPS_DIR to tempfile.gettempdir

So it fallbacks sensibly to environment variables TMPDIR, TEMP or TMP or
a sensisble platform default like '/tmp' while still being able to be
overriden by env TEMPS_DIR only for this library.
Merge pull request #1 from yaccz/fix-tmpdir-fallback

Fallback TEMPS_DIR to tempfile.gettempdir