Rename function which collideswith native PHP function

Hi,

here's a second version for that change. Your assumption is wrong that "empty" is just a method name and that the collision with the normal PHP method name won't matter. When PHP parses this file it will create a parser error and the further code execution will stop.

The error (with PHP 5.4) in this case: ``Parse error: parse error, expecting `"identifier (T_STRING)"' in Dataface/Clipboard.php on line 125``

But in order to stay compatible - here's a way to keep the interface to the outside available but rename the actual method.