ENH: fixed issue #2771 kill preivious storescp executables

@pieper
This fixes slicer bug 2771 [1] by killing all the instances of storescp before starting a new one. I tested it on Windows and Linux and works fine. 
Based on this change the text of toggleListener button is updated based on the process status not only user's click.

[1] http://na-mic.org/Mantis/view.php?id=2771