core: replaced SkipHistory exception by a key in mess_data:

SkipHistory was skipping all remaining triggers just to skip history, which is not the intented behaviour.
    Now history can be skipped by setting mess_data[u'history'] = C.HISTORY_SKIP, this way we won't skip importants triggers.
    When history is skipped, mess_data[u'extra'][u'history'] will be set to C.HISTORY_SKIP for frontends, so they can inform user that the message is not stored locally.

src/core/exceptions.py

@@ -108,10 +108,6 @@ class PermissionError(Exception):
     pass
 
 
-class SkipHistory(Exception): # used in MessageReceivedTrigger to avoid history writting
-    pass
-
-
 class ParsingError(Exception):
     pass