Update Grails.gitignore to include default files

Only files under /web-app/WEB-INF/classes (build output) should be ignored by Git. Grails generates a few default (unchanging) files (applicationContext.xml, sitemesh.xml, a bunch of *.tld) that need to be there for some plugins to function properly (e.g., grails war).
Merge pull request #926 from cknoblauch/patch-1

Update Grails.gitignore to include default files