add MANIFEST.in to force README.md to be packaged in `setup.py sdist`

The source tarball [on PyPI](https://pypi.python.org/packages/source/p/pymemcache/pymemcache-1.0.tar.gz) is not pip-installable, since it doesn't contain the README.md file referenced by setup.py. This adds a MANIFEST.in to explicitly list the files that should be packaged, and bumps version to 1.0.1.