Add git-remote-branches command

Credit:
http://www.commandlinefu.com/commands/view/2345/show-git-branches-by-date-useful-for-showing-active-branches
Update to latest version of git_completion

Via https://github.com/git/git/blob/6ab4ae2b415c375170309c2b7ace0e4daa8d0215/contrib/completion/git-completion.bash
Update Atom version in config.cson
Use Atom as editor for Git operations