Bug in Interval serialization

```
NodaIntervalConverter doesn't work when deserializing an interval as part of a 
larger object.

Tests and fix here:
http://code.google.com/r/mj1856-noda-time/source/detail?r=f02a3babd4e3e4fdba1c88
81ae8b1d5598d277b5

Please Merge.  Thanks.
```

Original issue reported on code.google.com by `mj1856` on 11 Feb 2013 at 4:35
