Added a manifest file

Without this, a normal ``pip install flickrbackup`` fails because it
can't read ``README.rst``:

    ➜  ~  pip install flickrbackup
    Downloading/unpacking flickrbackup
    ...
            long_description=open('README.rst').read(),
        IOError: [Errno 2] No such file or directory: 'README.rst'
    ...

etc. Probably merits a version bump & re-publishing to PyPI.