Added code to have Fling try to match the System Look and Feel.

On non-Mac platforms, unless the user has a system-wide default LookAndFeel set, Fling uses the Java default (Metal) which is very old looking.

The added code sets the UI to the LAF matching the System OS, one mirroring GTK for Gnome, one resembling Windows, and leaves it the same for Apple.

![Before - with Metal LAF](https://f.cloud.github.com/assets/66931/1011651/f13be0ce-0b60-11e3-9413-20e8627725b4.png)
![After - on a Linux machine running Gnome](https://f.cloud.github.com/assets/66931/1011656/f6cdc688-0b60-11e3-94a7-407e09ff760f.png)