Allow offset to be specified in ZonedDateTime text handling

```
In order to reliably round-trip a ZonedDateTime around DST changes, we need to 
have a field specifier for the offset. So for example, a full round trip value 
might be:

    2013-01-16T14:21:03.1234567 Europe/London +00
```

Original issue reported on code.google.com by `jonathan.skeet` on 16 Jan 2013 at 2:21
