Add username to standard branch prefix

[b4mboo]

Whenever you want to clean up your repository's branches and run git review clean --all there are a couple of left over branches, which have commits that have not yet been merged to master. We are looking for an easy way to know who might still need that branch or whether the contents are obsolete. In order to identify the one who created that branch, you can of course take a look at the commits (and that is probably the only safe way to handle these things). However, it would be nice to have the git username in the branch name to tell the respective team member to take care of that.

The idea is to simply put the git username into the prefix, which git-review uses to identify review branches. E.g. review_130514_b4mboo_feature_name.