[FEATURE] Add slot annotation for automatic wiring

This change adds an Slot annotation that can be used
to annotate method to wire them to existing signals
without the need to manually wire them in the bootstrap.

Example:

    /** 
     * @param Comment $comment
     * @return void
     * @Flow\Slot(class="Some\Package\Controller\CommentController", signal="commentCreated")
     */
    public function sendNewCommentNotification(Comment $comment) {
        ...
    }

Change-Id: I015bdc6bb5b7eba756765deb3ae76e615dc7de7f
Releases: master