wip: resellers and limits

I've finally started to rethink the database structure. The goal I'm trying to reach: only use User objects to manage permissions and accesses to the webui and associate other objects to them if needed (like mailboxes).

see #105

lib/exceptions.py

@@ -0,0 +1,18 @@
+# coding: utf-8
+"""
+:mod:`exceptions` --- Custom Modoboa exceptions
+-----------------------------------------------
+
+"""
+from django.utils.translation import ugettext as _
+
+class ModoboaException(Exception):
+    pass
+
+class PermDeniedException(ModoboaException):
+    def __init__(self, msg):
+        self.msg = msg
+
+    def __str__(self):
+        return _("Permission denied: %s" % self.msg)
+