Fixes #1485

 * Adding XML export of multi dim arrays
 * Adding metadata for all plugins that define the API functions, metrics, dimensions, category, report name, metrics translations
 * Could well be extended to include 100% of Piwik report metadata later
 * cleaning up Goals API returned values to ensure consistency with other responses in piwik APIs

git-svn-id: http://dev.piwik.org/svn/trunk@2549 59fd770c-687e-43c8-a1e3-f5a4ff64c105