Add date interpolation and tests

This enhances the built-in interpolation routine to correctly handle Date objects.

This is how it can be used:
``` javascript
> date1 = new Date(2010, 1, 0)
Sun Jan 31 2010 00:00:00 GMT-0800 (PST)
> date2 = new Date(2014, 1, 0)
Fri Jan 31 2014 00:00:00 GMT-0800 (PST)
> d3.interpolate(date1, date2)(0.3)
Thu Apr 14 2011 08:12:00 GMT-0700 (PDT)
```