Using setInterval() instead of setTimeout()

Using ```setInterval()``` better in case when ```editor.document``` is long or editor has to many [CKEDITOR.editor.dataFiltered](http://docs.ckeditor.com/#!/api/CKEDITOR.editor-event-dataFiltered) rules. 

As a result ```editor.document.getBody()``` could not be exist at ```observer.observe()``` call time, nothing to observe. ```setInterval()``` is kinde of listener, which waits until ```editor.document``` becomes an object and after that ```observer.observe()``` will be called.