Date with min and max rules

min and max comparison currently does not support Date using following code:
```javascript
var d = ko.observable(new Date(2012, 03, 04)).extend({ min: new Date(2010, 02, 02) });
```
because it was clearly written with HTML's input in mind where all values are passed as strings. Above code currently is always not valid.

We use special binding which translates value to/from date (with conjunction with jquery datepicker) so that in js we always deal with date.