Adding 'headless' java param to spoon runner

Currently, the spoon runner shows at OS X's dock while running as shown at the screenshot below (and probably other OS's UI too).
This patch adds the "headless" param to the java command line to prevent this behavior.

![screen shot 2013-05-14 at 2 07 06 pm](https://f.cloud.github.com/assets/21069/537582/a8eff114-c175-11e2-8da6-f67704bb02c1.png)