Initial work on fix for #424 - TypeError: can't pickle _thread.lock objects.

Made `Broker` and `Queue` classes pickable which fixes the the problem with new default `spawn` proces start method.

django_q/queues.py

@@ -54,9 +54,15 @@ class Queue(multiprocessing.queues.Queue):
             super(Queue, self).__init__(
                 *args, ctx=multiprocessing.get_context(), **kwargs
             )
-
         self.size = SharedCounter(0)
 
+    def __getstate__(self):
+        return super(Queue, self).__getstate__() + (self.size, )
+
+    def __setstate__(self, state):
+        super(Queue, self).__setstate__(state[:-1])
+        self.size = state[-1]
+
     def put(self, *args, **kwargs):
         super(Queue, self).put(*args, **kwargs)
         self.size.increment(1)