Don't check style in resources that are not on the class path

Fixed #10. Filter out resources that are not on the class path
of a Java project (class path includes source path). Even though
there are convenient methods on `ScalaProject`, I relied on
`JavaCore` so that I don't introduce a dependency on Scala IDE.

We should probably filter our `derived` resources as well, to provide a poor man's filtering mechanism. I imagine source generators might be adding things to the source path that violate style rules, but that aren't under the control of the programmer. That should be another ticket.