continue breaking down the epic autoload module

Add 2 new classes and move code into them from autoload:

* NERDTree. Each nerdtree buffer now has a NERDTree object that holds
  the root node and will old other util functions
* UI. Each NERDTree object holds a UI object which is responsible for
  rendering, getting the current node, etc

Still a fair few methods to sort through in autoload (many of which will
end up in the above classes) - need sleep though.

lib/nerdtree/nerdtree.vim

@@ -0,0 +1,23 @@
+"CLASS: NERDTree
+"============================================================
+let s:NERDTree = {}
+let g:NERDTree = s:NERDTree
+
+function! s:NERDTree.ForCurrentBuf()
+    return b:NERDTree
+endfunction
+
+function! s:NERDTree.New(path)
+    let newObj = copy(self)
+    let newObj.ui = g:NERDTreeUI.New(newObj)
+    let newObj.root = g:NERDTreeDirNode.New(a:path)
+
+    return newObj
+endfunction
+
+"FUNCTION: s:NERDTree.render() {{{1
+"A convenience function - since this is called often
+function! s:NERDTree.render()
+    call self.ui.render()
+endfunction
+