comp AP gateway: signature checking, caching and threads management:

- HTTP signature is checked for incoming messages
- AP actor can now be followed using pubsub subscription. When following is accepted, the
  node is cached
- replies to posts are put in cached pubsub comment nodes, with a `comments_max_depth`
  option to limit the number of comment nodes for a root message (documentation will come
  to explain this).

ticket 364

sat/core/exceptions.py

@@ -123,6 +123,11 @@ class ParsingError(Exception):
     pass
 
 
+class EncryptionError(Exception):
+    """Invalid encryption"""
+    pass
+
+
 # Something which need to be done is not available yet
 class NotReady(Exception):
     pass