Allow to zip empty directories

[ael-code]

At the moment the library don't allow add empty directory into the zipped stream.

There is also a TODO comment in the code:
https://github.com/allanlei/python-zipstream/blob/master/zipstream/__init__.py#L214

@allanlei do you have any plan on this? I really would like to help and I've read the code, but I think that I should know the zip structure details in order to implement this feature.