Initial suggestion for extension implmentation.

Usage example:

inside the `mkdocs_tree` extension, the includeme method will register for different event calllbacks:

    from mkdocs import events

    def printme(event):
         print event

    def includeme(config):
         events.register_callback(events.PreBuild, printme)
         events.register_callback(events.BuildPage, printme)

etc.