Make distribution `setup.py develop` compatible.

setuptools does not support non-root (non-empty) package names in the `package_dir` option when installing the distribution in development mode (`setup.py develop` or `pip install --editable`). So move lib/ contents under lib/ppymilter/ and use a root (empty) package name in the `package_dir` option.

https://bitbucket.org/pypa/setuptools/issue/230
https://bitbucket.org/tarek/distribute/issue/177
https://github.com/pypa/pip/issues/126