Support passing a generator

[pbauer]

Currently you cant create a ContentListing from a generator. Since in python3 many thing that previously were lists are now generators it would be usefull to allow passing a generator.
Obviously we cannot keep the generator but turn it into a sequence in __init__.py.

How about this?

    def __init__(self, sequence):
        if isinstance(sequence, GeneratorType):
            sequence = [i for i in sequence]
        self._basesequence = sequence