Media and other Handler objects: Improve toString method

In Media.java the toString() method currently returns this.url. As the url can be empty, this can result in a return null.
According to the javadoc it should return a textually representation of the object.

Especially if you are debugging, it is annoying to see that the current media-object is "null" (as the debugger calls toString). Therefor I'd love to see something more informative here.