MacPorts portfile and small bugfix

Hi,
Love the simplicity of the project. Thanks for putting it out there. I fixed a small compiler warning bug, and I added a MacPorts portfile. I haven't yet submitted it to MacPorts, I'll wait to see if you accept my pull request and advance the version number. If you accept this request, please edit and commit the Portfile with the new version number on the "version" and "git.branch" lines. Thanks!

Ross