Create soft dependency on logbook.

rq/daemon.py

@@ -1,4 +1,7 @@
-from logbook import Logger
+try:
+    from logbook import Logger
+except ImportError:
+    from logging import Logger
 from .worker import Worker
 
 def run_daemon(queue_keys, rv_ttl=500):

rq/worker.py

@@ -3,7 +3,10 @@ import os
 import random
 import time
 import procname
-from logbook import Logger
+try:
+    from logbook import Logger
+except ImportError:
+    from logging import Logger
 from pickle import loads, dumps
 from .queue import Queue
 from .proxy import conn