fix(searchEnabled): watch evaluated attribute value

`searchEnabled` was not reacting to changes in it's value.
  
This was because searchEnabled has a scope.$watch on it but searchEnabled
was set as a property on $select not the scope and the watch would never 
fire.  

This commit now changes the watch so it the attrs.searchEnabled for changes 
and properly reacts to changes in it's value.

Closes #505

src/uiSelectDirective.js

@@ -74,16 +74,15 @@ uis.directive('uiSelect',
           });
         }
 
-        scope.$watch('searchEnabled', function() {
-            var searchEnabled = scope.$eval(attrs.searchEnabled);
-            $select.searchEnabled = searchEnabled !== undefined ? searchEnabled : uiSelectConfig.searchEnabled;
+        scope.$watch(function () { return scope.$eval(attrs.searchEnabled); }, function(newVal) {
+          $select.searchEnabled = newVal !== undefined ? newVal : uiSelectConfig.searchEnabled;
         });
 
         scope.$watch('sortable', function() {
             var sortable = scope.$eval(attrs.sortable);
             $select.sortable = sortable !== undefined ? sortable : uiSelectConfig.sortable;
         });
-        
+
         attrs.$observe('limit', function() {
           //Limit the number of selections allowed
           $select.limit = (angular.isDefined(attrs.limit)) ? parseInt(attrs.limit, 10) : undefined;