Added a method so you can know whether an element is empty

Hi, working with your library (actually with another one that in turn used yours). I ran into the problem that I needed a way to differenciate
 this
    <tag attr='value'/>
from this
    <tag><attr>value</attr></tag>
My first approach was to, instead of returning the attributes as the value when an empty node is found, change it to a "" and monkeypatch it with an attributes method, pretty much the same as done in text nodes. Then I realized that this would break the current code written with the library pretty much more than expected. Either way, I ended up writing a patch that marks as empty the values of the empty nodes so a simple test can be done to overcome the problem then I had without breaking backward compatibility

Thanks for the library, Jose Fernández.