Code review comment for lp://staging/~xaav/bzr/tarball-generator

So, the last hurdles seem to be:
- executing the contributor agreement: http://www.canonical.com/contributors
- adding news entries in doc/en/release-notes and doc/en/whats-new

I've fixed some more PEP8 issues at lp:~vila/bzr/tarball-generator, feel free to merge them in your branch.

I've also added 'while the actual export is written to fileobj.' to the get_export_generator doc string as I had to read the code to understand why the generators were yielding None.

This addition may not be true for all generators but you seem to have mentioned the exceptions already.