Pyfs expose mocks3

Hi,

This includes the other changes that are in the other pull request (mockredispy), and it converts this into a module for pyfs.  This way any filesystem that pyfs can expose can be turned into the data store behind mock-s3.  You can see the feature bug at:
http://code.google.com/p/pyfilesystem/issues/detail?id=182

This can be used separately from pyfs if you just keep the mock_s3.py named the same and roll back the rename of the jinja2 path.  This way you can just consume the pyfs filesystems without having to integrate with the project.  Either way, it would be a nice general feature for mock_s3 even if pyfs doesn't want to include this as a module of their project.

Thanks for such a nice little module with a great usable licence.  This is great!  I think the next TODO is to figure out if authentication would be worth it.