Make compatible with oauth2client v2.0.0

The v2.0.0 release moved a few modules to a subpackage of oauth2client
called contrib. Among these was the locked_file module necessary for
credential storage.

Sample code for Django, App Engine and Service Accounts needed updates
as well.
diff --git a/samples/appengine/main.py b/samples/appengine/main.py
index 3d8587b..d2a3223 100644
--- a/samples/appengine/main.py
+++ b/samples/appengine/main.py
@@ -31,8 +31,8 @@
 import pickle
 
 from googleapiclient import discovery
-from oauth2client import appengine
 from oauth2client import client
+from oauth2client.contrib import appengine
 from google.appengine.api import memcache
 
 import webapp2