[debian] Update d/rules and switch to non-native package

- Cleaned debian/rules by using python2 and sphinxdoc
- Added debian/watch file
- Override get-orig-source (remove .git and debian dir. from .orig tgz)

https://wiki.debian.org/DebianMentorsFaq#What_is_the_difference_between_a_native_Debian_package_and_a_non-native_package.3F
https://wiki.debian.org/DebianMentorsFaq#What.27s_wrong_with_upstream_shipping_a_debian.2F_directory.3F