added git-flow

No way for testing, but git-flow has only "git | git-core" as dependencies, so seems to be easy to merge.

    $ apt-cache show git-flow
    Package: git-flow
    Version: 0.4.1-2
    Installed-Size: 110
    Maintainer: Gergely Nagy <algernon@madhouse-project.org>
    Architecture: all
    Depends: git | git-core
    Description-en: Git extension to provide a high-level branching model
     A set of scripts that provide high-level repository operations for
     managing feature/release/hotfix branches in a Git repository,
     particularly suited to be utilised to follow Vincent Driessen's
     branching model, described at
     <http://nvie.com/posts/a-successful-git-branching-model/>.
    Homepage: https://github.com/nvie/gitflow/wiki
    Description-md5: 67abe51a90fde2614c903ffb227e0dc3
    Section: vcs
    Priority: extra
    Filename: pool/main/g/git-flow/git-flow_0.4.1-2_all.deb
    Size: 27458
    MD5sum: c86dd0d6958e16e420ab2d4a75eccb03
    SHA1: 493b58f81b159af46ffd51c1671d4c47106ef705
    SHA256: ba0f425e9b4acdeef6dae004c39b3032df5cacdfdfb6619273baba759366257b

    $ apt-cache policy git-flow
    git-flow:
      Installed: 0.4.1-2
      Candidate: 0.4.1-2
      Version table:
     *** 0.4.1-2 0
            500 http://ftp.es.debian.org/debian/ wheezy/main i386 Packages
            100 /var/lib/dpkg/status

Fixing #22