This uses a weak reference to the notifier in _SysProcessEvent and adds
a `__del__` method for Notifier.

This prevents pyinotify from leaking inotify file descriptors
(instances) and its watches.

Fixes seb-m#95

I have used the following script to test it:

    from __future__ import print_function

    import gc
    import os

    from pyinotify import Notifier, WatchManager

    def has_inotify_fds():
        "Does the current process have inotify fds?"
        r = 0
        for x in os.walk('/proc/{}/fd'.format(os.getpid())):
            for y in x[2]:
                path = os.path.join(x[0], y)
                if os.path.exists(path):
                    if 'inotify' in os.path.join(os.readlink(path)):
                        r += 1
        return r

    gc.disable()

    wm = WatchManager()
    n = Notifier(wm)
    assert has_inotify_fds() == 1

    # Stopping multiple times should work:
    n.stop()
    assert has_inotify_fds() == 0
    n.stop()

    gc.collect()
    # XXX: needs a new WatchManager, because the previous fd gets closed by stop!
    wm = WatchManager()
    n = Notifier(wm)
    assert has_inotify_fds() == 1

    # Collect anything.
    gc.collect()
    assert n in gc.get_objects()

    n = None
    assert gc.garbage == []
    assert not any(isinstance(x, Notifier) for x in gc.garbage)
    assert not any(isinstance(x, Notifier) for x in gc.get_objects())

    assert has_inotify_fds() == 0

This uses a weak reference to the notifier in _SysProcessEvent and adds
a `__del__` method for Notifier.

This prevents pyinotify from leaking inotify file descriptors
(instances) and its watches.

Fixes seb-m#95

I have used the following script to test it:

    from __future__ import print_function

    import gc
    import os

    from pyinotify import Notifier, WatchManager

    def has_inotify_fds():
        "Does the current process have inotify fds?"
        r = 0
        for x in os.walk('/proc/{}/fd'.format(os.getpid())):
            for y in x[2]:
                path = os.path.join(x[0], y)
                if os.path.exists(path):
                    if 'inotify' in os.path.join(os.readlink(path)):
                        r += 1
        return r

    gc.disable()

    wm = WatchManager()
    n = Notifier(wm)
    assert has_inotify_fds() == 1

    # Stopping multiple times should work:
    n.stop()
    assert has_inotify_fds() == 0
    n.stop()

    gc.collect()
    # XXX: needs a new WatchManager, because the previous fd gets closed by stop!
    wm = WatchManager()
    n = Notifier(wm)
    assert has_inotify_fds() == 1

    # Collect anything.
    gc.collect()
    assert n in gc.get_objects()

    n = None
    assert gc.garbage == []
    assert not any(isinstance(x, Notifier) for x in gc.garbage)
    assert not any(isinstance(x, Notifier) for x in gc.get_objects())

    assert has_inotify_fds() == 0