Send WikiReadyEvent event when a wiki is ready to be used

We already have ApplicationStartedEvent and ApplicationReadyEvent (when DB is ready). We also need an event when the wiki is ready since in s a multiwiki environment when you create a new wiki for example, you might need to init some stuff (like initialize mandatory classes). It's also needed the first time a request comes in for a given wiki.

See http://xwiki.markmail.org/thread/tykv3w243gjfvg4p for more details