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.