allow people stuck with Java 6 to use SwingWizard

SwingWizard currently does not run on Java 6. I know Java 8 has been out for a bit, but my client is still stuck with Java 6. Luckily, SwingWizard compiles nicely with Java 6, and I hope you agree there isn't a strong reason at this point to tie it to Java 7.

Please accept this patch to change the source level/target to Java 6.