include tests data in .egg
include __init__.py in gratelpy/mechanisms

not exactly sure if this is needed but I've seen this being suggested to
improve our chances of setuptools.setup actually copying all indicated
data_package to the intended install directory
use pyin.resource_path for packaged scripts
a recent version of setuptools.setup is necessary for package_data and entry_point
get package_data right on Windows
entry_points would not work on Windows so let us just hack scripts
use os.path.join to make certain paths make sense on all OS