partial_length.gpg test data file is missing from tarball

Running the tests from the source tarball fails with this:

```
File "python-pgpdump/pgpdump/test.py", line 435, in test_parse_partial_length
    rawdata = self.load_data('partial_length.gpg')
  File "python-pgpdump/pgpdump/test.py", line 102, in load_data
    self.assertTrue(os.path.exists(full_path))
```

It probably needs to be added to the MANIFEST or something like that.
