fix(datepicker): Apply custom class to month

Closes angular-ui#3863

template/datepicker/month.html

@@ -8,7 +8,7 @@
   </thead>
   <tbody>
     <tr ng-repeat="row in rows track by $index">
-      <td ng-repeat="dt in row track by dt.date" class="text-center" role="gridcell" id="{{::dt.uid}}">
+      <td ng-repeat="dt in row track by dt.date" class="text-center" role="gridcell" id="{{::dt.uid}}" ng-class="::dt.customClass">
         <button type="button" style="width:100%;" class="btn btn-default" ng-class="{'btn-info': dt.selected, active: isActive(dt)}" ng-click="select(dt.date)" ng-disabled="dt.disabled" tabindex="-1"><span ng-class="::{'text-info': dt.current}">{{::dt.label}}</span></button>
       </td>
     </tr>