remote-bzr: Set Author if available.

First off, thank you so much for this bzr bridge. After some pretty large merges, neither bzr fastexport nor good old 'tailor' were able to convert our bzr repository anymore (cedet.sf.net), but your bridge worked like a charm.

The only thing that was missing is to extract the author's name, which might be different from the committer, which is what this commit adds.

Thanks again,
David