Added the ability to pass a function as the mask callback for the express middleware.
if you have a complex item that needs masking  you can pass a
callback funtion that will be invoked before your object is serialized to json.

 The callback must have the following signature.
 function(obj, mask, done);

     express.use(mongooseMask(function(value, mask, done){
             var masked = mask(value, ['_id', '__v']);
             if(masked.data){
                 masked.data = mask(value.data, ['_id', '__v']);
             }
             done(null, masked);
         }));