Make the behavior of window splits consistent (#1035)

* add option to spawn split from previous window

* Update some comments and remove debugging statement.

* Simplify the code, and make it respect NERDTreeWinPos and 'splitbelow'.

* Better logic for getting NERDTree window number when not active window.

* Use common functions for splitting windows. Make splitting respect user.

The placement of split windows now respects the user's choices for the
splitright and splitbelow settings, and the g:NERDTreeWinPos variable.

* Update version number in change log.

lib/nerdtree/nerdtree.vim

@@ -148,12 +148,19 @@ function! s:NERDTree.GetWinNum()
         return bufwinnr(t:NERDTreeBufName)
     endif
 
+    " If WindowTree, there is no t:NERDTreeBufName variable. Search all windows.
+    for w in range(1,winnr('$'))
+        if bufname(winbufnr(w)) =~# '^' . g:NERDTreeCreator.BufNamePrefix() . '\d\+$'
+            return w
+        endif
+    endfor
+
     return -1
 endfunction
 
 "FUNCTION: s:NERDTree.IsOpen() {{{1
 function! s:NERDTree.IsOpen()
-    return s:NERDTree.GetWinNum() != -1 || bufname('%') =~# '^' . g:NERDTreeCreator.BufNamePrefix() . '\d\+$'
+    return s:NERDTree.GetWinNum() != -1
 endfunction
 
 "FUNCTION: s:NERDTree.isTabTree() {{{1