add proper events and make the notifier class generic

Expand the event system to have explicit Event objects and potentially
many Notifiers. Previously they was only one notifier and one (implied)
event.

A lot of this is stolen from #358.

lib/nerdtree/tree_dir_node.vim

@@ -252,7 +252,7 @@ function! s:TreeDirNode._initChildren(silent)
             try
                 let path = g:NERDTreePath.New(i)
                 call self.createChild(path, 0)
-                call g:NERDTreeRefreshNotifier.NotifyListeners(path)
+                call g:NERDTreePathNotifier.NotifyListeners('init', path, {})
             catch /^NERDTree.\(InvalidArguments\|InvalidFiletype\)Error/
                 let invalidFilesFound += 1
             endtry