make email the only one user index by default

src/kvs_user.erl

@@ -10,6 +10,6 @@ metainfo() ->
 %        #table{name=user2,container=feed,fields=record_info(fields,user2)},
         #table{name=group,container=feed,fields=record_info(fields,group)},
         #table{name=user,container=feed,fields=record_info(fields,user),
-                 keys=[facebook_id,googleplus_id,twitter_id,github_id]}
+                 keys=[email]}
     ]}.