Refresh SignedJwtAssertionCredentials w/Storage.

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

Fixes issue #160.
diff --git a/oauth2client/appengine.py b/oauth2client/appengine.py
index b484c1e..ca2ff15 100644
--- a/oauth2client/appengine.py
+++ b/oauth2client/appengine.py
@@ -42,6 +42,9 @@
 from google.appengine.ext.webapp.util import login_required
 from google.appengine.ext.webapp.util import run_wsgi_app
 
+
+logger = logging.getLogger(__name__)
+
 OAUTH2CLIENT_NAMESPACE = 'oauth2client#ns'
 
 
@@ -148,7 +151,7 @@
 
   # For writing to datastore.
   def get_value_for_datastore(self, model_instance):
-    logging.info("get: Got type " + str(type(model_instance)))
+    logger.info("get: Got type " + str(type(model_instance)))
     cred = super(CredentialsProperty,
                  self).get_value_for_datastore(model_instance)
     if cred is None:
@@ -159,7 +162,7 @@
 
   # For reading from datastore.
   def make_value_from_datastore(self, value):
-    logging.info("make: Got type " + str(type(value)))
+    logger.info("make: Got type " + str(type(value)))
     if value is None:
       return None
     if len(value) == 0:
@@ -172,7 +175,7 @@
 
   def validate(self, value):
     value = super(CredentialsProperty, self).validate(value)
-    logging.info("validate: Got type " + str(type(value)))
+    logger.info("validate: Got type " + str(type(value)))
     if value is not None and not isinstance(value, Credentials):
       raise db.BadValueError('Property %s must be convertible '
                           'to a Credentials instance (%s)' %