Pull Request: OffsetDateTime.SwitchOffset

```
This seems missing from the API.  Please merge.  Thanks.

https://code.google.com/r/mj1856-noda-time/source/detail?r=a487d4ab0c92fc34f42e9
d4f97e00513fce2510a
```

Original issue reported on code.google.com by `mj1856` on 8 Sep 2013 at 11:01
