feat($ionicModal): close on backdrop click on desktop/tablet

Closes #1087

js/angular/directive/modal.js

@@ -8,7 +8,7 @@ IonicModule
     restrict: 'E',
     transclude: true,
     replace: true,
-    template: '<div class="modal-backdrop">' +
+    template: '<div class="modal-backdrop" ng-click="modal.hide()">' +
                 '<div class="modal-wrapper" ng-transclude></div>' +
               '</div>'
   };

scss/_modal.scss

@@ -51,7 +51,8 @@
 .modal-open {
   pointer-events: none;
 
-  .modal {
+  .modal,
+  .modal-backdrop {
     pointer-events: auto;
   }
 }