Add git init to `ember new`

This was my thought as well @ccoenen.  I was kinda hoping that people programming on windows might use [mysysgit](http://msysgit.github.io/) which comes with a bash emulator.  But you're right there is probably a non trivial segment using cmd.exe or powershell.  Eitherway, `git --version` should do the trick in this case.