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.)