Support nested fields for the monitoring.

In some cases are need to use that construction
```Haxe
DC.monitorField(object, 'field.nestedField', 'someProp');
```
because 'field' for some reasons can be null:
```Haxe
object.field = null;
// ...
DC.monitorField(object.field, 'nestedField', 'someProp');    // log error
```