fix(datepicker): ng-model value can be a timestamp

Accept a number of milliseconds since 01.01.1970 as a valid value
for `ng-model`

Closes angular-ui#2345

Author: khashayar

Diff for: src/datepicker/datepicker.js

@@ -502,6 +502,11 @@ function ($compile, $parse, $document, $position, dateFilter, dateParser, datepi
       }
 
       function parseDate(viewValue) {
+        if (angular.isNumber(viewValue)) {
+          // presumably timestamp to date object
+          viewValue = new Date(viewValue);
+        }
+
         if (!viewValue) {
           ngModel.$setValidity('date', true);
           return null;