Removed explicit dependency to logging plugin so people can change the logging framework

- resubmitted long outstanding pull request to this forked project for merge. Patch originally by fterrier. See https://github.com/Grailsrocks/grails-cache-headers/pull/1

When following the instructions in the Grails guide (http://grails.org/doc/2.0.x/guide/conf.html#logging), run-app fails because of that explicit dependency. Note: This is needed e.g. if you want to run build a grails app for Glassfish.

To reproduce, add to an app using the cache-headers plugin:
...
inherits("global") {
excludes "grails-plugin-logging", "log4j"
}
…

and then grails run-app