Fix NERDTree opening with the wrong size. (#1008)

* Fix NERDTree opening with the wrong size.

There were two commands that seemed to be causing the NERDTree window to
open in exactly half the screen width. They are:

- silent! execute 'edit ' . t:NERDTreeBufName        in _createTreeWin()
- setlocal nobuflisted                         in _setCommonBufOptions()

These commands were reseting the width of NERDTree. The solution I chose
was to resize the window after creating the new window, and then to set
winfixwidth before setting the other common buffer options.

* Update change log and version number.

autoload/nerdtree.vim

@@ -4,7 +4,7 @@ endif
 let g:loaded_nerdtree_autoload = 1
 
 function! nerdtree#version()
-    return '5.1.1'
+    return '5.1.2'
 endfunction
 
 " SECTION: General Functions {{{1