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.)
If this test fails, I'd assume that there will be runtime issue as well when rendering a pie chart. Would you mind fixing the test as well?