Add git init to `ember new`

I think `git --version` is the way to go. 

 Alternatively, I could use the bash/zsh builtin `command -v git` which will return the string "git" or nothing depending on availability.  That should work for most people on windows and bash/zsh, but I'm not sure it's availability would be wide enough for people with other setups.

If we do go with `git --version` from exec the fact that it doesn't error should be enough (/cc @stefanpenner).   I'm not sure we could get more specific with the error handling.  But maybe I'm missing something obvious.

I think this is close to needing a rebase to clean it up some.