remote-bzr: include authors field in bzr commits

Trivial patch. It's already parsed from the git input, just needed to add it to revprops.

(I needed to remove `.git/bzr/<remote>/` to be able to get this revprop added to commits I pushed before, without this patch)

Having to send this pull request reminds me of this particular part of the [blog post][1] that introduces these helpers:

> So, are there any drawbacks? Surely some information must be lost.
> Nope, not really. At least not when using the hg-git compat mode, because all the extra information is saved in the commit messages.

I knew some day I would find some missing metadata. (But, TBH, other than this detail, it's been wonderful)

[1]: http://felipec.wordpress.com/2012/11/13/git-remote-hg-bzr-2/