Added 'per page' encoding (see GPGOOGLEVISUALIZATIONAPI-14)

Map rendering under Grails 2.3+ escapes quotes in strings, so the map doesn't render. As discussed in http://grails.org/doc/latest/guide/security.html#xssPrevention, escaping XSS prevention can be handled on a single page with the given page directive.

(Note that I'm not sure how to test plugins, so I haven't checked that the existing tests all pass, but the plugin now works for my application.)
I'd like to keep the Tomcat plugin as it is used to test all visualizations with a help of a [GSP](https://github.com/bmuschko/grails-google-visualization/blob/master/grails-app/views/index.gsp). Adding this plugin in `BuildConfig.groovy` however blows up the build. Can you look into making this work again?