Return a URL for routing between two (lat, lng) at a datetime.

def LatLngsToGoogleUrl(source, destination, dt):
  """Return a URL for routing between two (lat, lng) at a datetime."""
  params = {"saddr": FormatLatLng(source),
            "daddr": FormatLatLng(destination),
            "time": dt.strftime("%I:%M%p"),
            "date": dt.strftime("%Y-%m-%d"),
            "dirflg": "r",
            "ie": "UTF8",
            "oe": "UTF8"}
  url = urlparse.urlunsplit(("http", "maps.google.com", "/maps",
                             urllib.urlencode(params), ""))
  return url