python: Remove Messages().__len__

Messages.__len__() exhausted the iterator and list() inherently calls
len(), so we could not invoke list(msgs) without getting errors. Fix
this by implementing __nonzero__ but removing __len__ on Messages.

Use Query.count_messages() or len(list(msgs)) if you need to know the
number.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>