Use io.open to open streams in PackageResource.open for filesystem resources

It seems the default filestream opened by pkg_resources.resource_stream
is not always usable for TextIOWrapper, even if a file resource is
opened.
Add *.pxd files to source distribution

This is important when the package is first bundled as a source
distribution and then installed:

    python setup.py sdist
    pip install dist/PyMAD-<version>.tar.gz