Hubflow commands added

Added commands, menu items, and implementation for [HubFlow](http://datasift.github.io/gitflow/). Commands implemented are:

    feature   Manage your feature branches.
    release   Manage your release branches.
    hotfix    Manage your hotfix branches.
    push      Push the changes from your current branch (plus any new tags) back upstream.
    pull      Pull upstream changes down into your master, develop, and current branches.
    update    Pull upstream changes down into your master and develop branches.