Set application icon to logo_splash.png

Currently Nightcode app icon is displayed as the Java icon, which is generic and boring. This PR sets the app icon to the `logo_splash.png` image. Tested on Linux and OS X Lion. OS X needs special handling as described at the reference URL below.

Reference: http://stackoverflow.com/questions/11253772/setting-the-default-application-icon-image-in-java-swing-on-os-x