Bump to version 0.3.1

It's been two years since 0.3.0 and there have been a few
small changes since then (including 5.2 compat).  Cutting
a new version allows other projects (and rocks) to pull
down additions easier.

Also adding a rockspec for 0.3.1 (tag 0.3.1 should be made
on the origin repo so rocks can pull properly).