A couple of cleanup commits

@marcpalmer here's some commits that do a few (very minor) cleanups to the grails-cache-headers plugin.

* Remove direct dependency on "logging" core plugin - this was causing issues for us when switching away from the default Grails logging implementation, as it brings in the log4j binding for slf4j.
* Removed tomcat/hibernate dependencies from application.properties - they're not needed for the plugin and thus shouldn't bring transitive baggage into projects that use grails-cache-headers plugin.
* Checked in grails-app/conf/.empty, as empty directories don't get staged in git index, but grails test-app freaks out if the conf directory doesn't exist.

Let me know if you have any queries!