add support for time.Duration

Hi Dmitry,

We frequently use properties files to configure timeouts.  Utlimately we want to convert the values into a `time.Duration`.  It turns out there is already a nice [time.ParseDuration()](http://golang.org/pkg/time/#ParseDuration) function to do all the heavy lifting.  I think it would be a great addition to your package if you would include this patch to add direct support for `time.Duration`.

thanks!
mike