Add a conversion from Period to Duration

```
Periods which only contain fixed-length fields should be convertible to 
Duration, in a way which assumes a day is only ever 24 hours:

    // This will only use time fields, which are okay
    Period period = Period.Between(time1, time2);
    Duration duration = period.ToDuration();

```

Original issue reported on code.google.com by `jonathan.skeet` on 18 Apr 2012 at 6:04
