CDK-546: Initial version of a Spark demo.

That exclusion is in the app parent. I had to re-add it here because I had to redeclare spark to come first. I could put spark first in the parent pom to compensate. My only concern is if you're not building a spark app then the version may not be what you want.

Actually, I could put this into DependencyManagement in the parent. Someone building a spark app would still need to add Spark to the pom like this, but they wouldn't need the exclusion or the version number.