Refresh SignedJwtAssertionCredentials w/Storage.

Reviewed in http://codereview.appspot.com/6346086/.

Fixes issue #160.
diff --git a/oauth2client/crypt.py b/oauth2client/crypt.py
index 3df861d..4204417 100644
--- a/oauth2client/crypt.py
+++ b/oauth2client/crypt.py
@@ -24,6 +24,8 @@
 from anyjson import simplejson
 
 
+logger = logging.getLogger(__name__)
+
 CLOCK_SKEW_SECS = 300  # 5 minutes in seconds
 AUTH_TOKEN_LIFETIME_SECS = 300  # 5 minutes in seconds
 MAX_TOKEN_LIFETIME_SECS = 86400  # 1 day in seconds
@@ -161,7 +163,7 @@
   signature = signer.sign(signing_input)
   segments.append(_urlsafe_b64encode(signature))
 
-  logging.debug(str(segments))
+  logger.debug(str(segments))
 
   return '.'.join(segments)