Explode iterator

Hi 

I'm not sure if this is something you like, but I thought it would be fun to see if I could implement the explode function as an iterator.  It slightly breaks your model, but I think it works nicely.  I have added an example of how it works to the `demo.php` file.

    /**
     * Can be used as an iterator after call to explode function
     */ 
     $string = new SimpleString('Lorem,ipsum,dolar,sit,amet,lorem,ipsum');
     foreach($string->explode(',') as $key => $value)
     {
       echo "<BR>Explode Value $key = $value";
     }

I know you want to keep your design really simple and every function should return a string so if you feel that this change doesn't fit in that's cool.  If you think its good then I'd like any  feedback to make it better, also I can add a test and documentation.