Connect to S3 and download the metadata for each file in all buckets
    specified and cache the data to disk.

def _init():
    '''
    Connect to S3 and download the metadata for each file in all buckets
    specified and cache the data to disk.
    '''
    cache_file = _get_buckets_cache_filename()
    exp = time.time() - S3_CACHE_EXPIRE

    # check mtime of the buckets files cache
    metadata = None
    try:
        if os.path.getmtime(cache_file) > exp:
            metadata = _read_buckets_cache_file(cache_file)
    except OSError:
        pass

    if metadata is None:
        # bucket files cache expired or does not exist
        metadata = _refresh_buckets_cache_file(cache_file)

    return metadata