Make .sh wrappers a bit friendlier

* Try using python2 first. On many systems, /usr/bin/python is a symlink to the actual python installation, which in some cases is python3. Since the python scripts used here aren't compatible with python3, we check for python2 first. The previous behavior here is preserved if neither python2 or python can be found, by simply echoing "python." (This could happen if the system does not have which installed)
* Add release.sh, wrapping release.py.
* Make install/install.sh executable by default, like the other wrappers.