Re-add MANIFEST.in post-0.3.1 release

For a local install (./setup.py install) the files listed
in setup.py suffice, but for sdist, files like scc_version.py
must also be listed in MANIFEST.in in order to be included.
Merge pull request #67 from joshmoore/manifest-in

Re-add MANIFEST.in post-0.3.1 release