For options.keep_author, update via post-commit rev-prop

For options.keep_author, update via post-commit rev-prop

Rather than using "svn commit --username %author%", mirror the
source author info by setting the "svn:author" revprop post-commit
the same as we do for "svn:date" (options.keep_date). This frees us
up to be able to use --username in the future for legitimate
source/target repo auth.

* svn2svn/run/svn2svn.py (commit_from_svn_log_entry): Update
  "svn:author" via post-commit revprop rather than --username.
* README.mkd: Reflect new "svn:author" handling.