Python wrappers for libsystemd API

CHANGES WITH 234:

        * Support for the new sd_is_socket_sockaddr added in systemd 233
          is added.

        * New id128 constants added in systemd 233 are included.

        * "Extra" fields for the log entry can be passed during
          LoggerAdapter initialization or for each message separately.
          Fields specified for the message have higher priority.

        * Small fixes and style tweaks all over.

        Contributions from: Park Jeongsoo, Jimmy Cao, Johannes
        Weberhofer, Mike Gilbert, Oleksii Shevchuk, Ville Skyttä,
        Wesley Bowman, Zbigniew Jędrzejewski-Szmek

CHANGES WITH 233:

        * Tests are fixed and should pass on various old and new
          systems alike.

        * journal.stream() can be used without arguments and defaults
          to LOG_LEVEL.

       Contributions from Robert James Hernandez,
       Zbigniew Jędrzejewski-Szmek

CHANGES WITH 232:

        * Wrappers for sd_journal_enumerate_unique, has_runtime_files,
          has_peristent_files.

        * sd_journal_open_directory_fd, sd_journal_open_files_fd can
          be used by passing file descriptors to the initializer as
          the path or files arguments.

          SD_JOURNAL_OS_ROOT flag is supported, and various flags may
          be passed to the constructor in combination with path or
          files arguments. All flags arguments are now passed through
          to the underlying libsystemd library functions, so which
          combinations are supported depends on that library.
          systemd 232 contains various fixes in this area.

        Contributions from: Benedit Morbach, Michael Biebl,
        Michael Herold, Mike Gilbert, Nir Soffer, Sebastian,
        Ville Skyttä, Zbigniew Jędrzejewski-Szmek

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
