Add Exo plugin to allow easy access to Exo's JCR storage

It's currently possible to call Exo services from within XWiki but it's not very easy. This plugin will make it extra easy to get a reference to a Exo JCR Session.

In the future, we should move all Exo features (like group/user authentication, etc) inside that plugin in order to decouple Exo from the core (see XWIKI-350 and XWIKI-856).