Update Grails.gitignore to include default files

[Grails](http://grails.org) generates a few default files under `/web-app/WEB-INF/` (`applicationContext.xml`, `sitemesh.xml`, a bunch of `*.tld`) that need to be present for some default targets to work. Only  should be ignored by Git.
I ran into this issue while trying to run the `grails war` target against a Grails 2.3.1 application that had this `.gitignore`, so I change it here to only ignore files under `/web-app/WEB-INF/classes` (build output) . See [here](https://stackoverflow.com/questions/14910388/grails-push-to-heroku-failed-applicationcontext-xml-doesnt-exist) for reference.