conditionally add tz and offset if they are not None

also use the new style formatting

e.g. something like

```
freq = ", offset={0}".format(self.offset.freqstr) if self.offset is not None else ""
tz = ", tz={0}".format(self.tz) if self.tz is not None else ""

"Timestamp('{stamp}'{zone}{freq}})".format(stamp=stamp,zone=zone,freq=freq)
```