fixes#5104, refs #4348, #2263
- Create a centralised event module
- Hook it up for posts, pages, tags and users
- Use it in sitemaps instead of direct method calls
- Use it for xmlrpc calls
- Check events are fired in model tests
- Update sitemap tests to work with new code
- Fix a bug where invited users were appearing in sitemaps
- Move sitemaps and xmlrpc into a directory together
