[vim plugin] fix tern#Shutdown for windows

os.kill complains about invalid handle (possibly process handle vs pid issue).

Use [taskkill](http://technet.microsoft.com/de-DE/library/cc725602.aspx) to kill both intermediate shell (from `shell=True` in `tern_startServer`) and its child process, tern server. Use `shell=True` again to prevent console window popup.