New preference "run.jdk.unsupported" to disable the warning message for unsupported JDK.

In order to test the latest processing with OpenJDK, I am heavily using the latest Processing with OpenJDK 7. Every time I launches Processing it shows a warning message saying I'm running it with an unsupported JDK. This message would be very useful for users, but annoying for me. I have added an option to disable the message.

I did not add a check box to the warning message dialog to true this option, because someone will turn the check box on without reading the message (it's human's natural behavior) and it will cause nightmare.