Python wrappers for libsystemd API

CHANGES WITH 231:

        * Various build fixes and cleanups: documentation can be
          generated with 'make sphinx-html', pip install works out
          of the box.

        * Tests that cover most of the python code were added. As a
          result, a bug in sd_is_mq was fixed in systemd 227.

        * Functions sd_pid_notify and sd_pid_notify_with_fds are now
          wrapped as optional arguments to notify(), when compiled
          against a new-enough libsystemd.

        Contributions from: David Strauss, Evgeny Vereshchagin,
        Jacek Konieczny, Jeroen Dekkers, Zbigniew Jędrzejewski-Szmek

CHANGES WITH 230:

        * python-systemd is again a separate project, after being part
          of systemd for many years.

        Contributions from: Dave Reisner, David Strauss,
        Evgeny Vereshchagin, Greg KH, Harald Hoyer, Jacek Konieczny,
        Jeroen Dekkers, Kay Sievers, Lennart Poettering, Lukas Nykryn,
        Marti Raudsepp, Richard Marko, Simon Farnsworth,
        Steven Hiscocks, Thomas Hindoe Paaboel Andersen, Ville Skyttä,
        Zbigniew Jędrzejewski-Szmek