bindings/python: Implement Message().get_filenames()

Message().get_filenames() will return a generator that allows to
iterator over the recorded filenames for a certain Message. Do ntoe that
as all generators, these are one-time use only. You will have to reget
them to perform various actions. So this works::

  len(Message().get_filenames())
  list(Message().get_filenames())
  for n in Message().get_filenames():
    print n

But this won't::

  names = Message().get_filenames()
  len(names) #uses up the iterator
  list(names) #outch, already used up...

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