Rectify ui-date, same ngModel linkage problems

thomas83: When two ui-dates share the same ngModel, and one of the dates change, the other does not get reflected with correct dateFormat (when you have a custom format of course).  This commit makes sure the format gets changed correctly. Also, when date is string when one ng-model w/ ui-date is binded with ng-model w/ ui-date, it causes an exception (even if you define date-date-format).  Thus here, we make sure date is a "Date" object here.  If not, we will do the conversion.