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/brokers/__init__.py

@@ -13,6 +13,14 @@ class Broker:
         self.cache = self.get_cache()
         self._info = None
 
+    def __getstate__(self):
+        return self.list_key, self._info
+
+    def __setstate__(self, state):
+        self.list_key, self._info = state
+        self.connection = self.get_connection(self.list_key)
+        self.cache = self.get_cache()
+
     def enqueue(self, task):
         """
         Puts a task onto the queue