Don't try to use dynamic tags files for non-writable directories (issue #29)

This fixes the problem described here:
#29

Author: xolox

README.md

@@ -91,7 +91,7 @@ By default `:UpdateTags` only writes to the global tags file, but it can be conf
 
 You can change the name of the tags file, the important thing is that it's relative to your working directory or the buffer (using a leading `./`). When `g:easytags_dynamic_files` is set to 1 the easytags plug-in will write to the first existing tags file seen by Vim (based on the ['tags' option] [tags_opt]). In other words: If a project specific tags file is found it will be used, otherwise the plug-in falls back to the global tags file (or a file type specific tags file).
 
-If you set `g:easytags_dynamic_files` to 2 the easytags plug-in will automatically create project specific tags based on the first name in the 'tags' option. This disables the global tags file and file type specific tags files.
+If you set `g:easytags_dynamic_files` to 2 the easytags plug-in will automatically create project specific tags based on the first name in the 'tags' option. In this mode the the global tags file or file type specific tags files are only used for directories where you don't have write permissions.
 
 The ['tags' option] [tags_opt] is reevaluated each time the plug-in runs, so which tags file is selected can differ depending on the buffer and working directory.
 

autoload/xolox/easytags.vim

@@ -3,7 +3,7 @@
 " Last Change: April 30, 2013
 " URL: http://peterodding.com/code/vim/easytags/
 
-let g:xolox#easytags#version = '3.1.7'
+let g:xolox#easytags#version = '3.1.8'
 
 call xolox#misc#compat#check('easytags', 2)
 
@@ -651,6 +651,13 @@ function! xolox#easytags#get_tagsfile() " {{{2
     let tagsfile = get(tagfiles(), 0, '')
   elseif dynamic_files == 2
     let tagsfile = xolox#misc#option#eval_tags(&tags, 1)
+    let directory = fnamemodify(tagsfile, ':h')
+    if filewritable(directory) != 2
+      " If the directory of the dynamic tags file is not writable, we fall
+      " back to a file type specific tags file or the global tags file.
+      call xolox#misc#msg#warn("easytags.vim %s: Dynamic tags files enabled but %s not writable so falling back.", g:xolox#easytags#version, directory)
+      let tagsfile = ''
+    endif
   endif
   " Check if a file type specific tags file is useful?
   if empty(tagsfile) && !empty(g:easytags_by_filetype) && index(xolox#easytags#supported_filetypes(), &ft) >= 0

doc/easytags.txt

@@ -229,8 +229,9 @@ file).
 
 If you set |g:easytags_dynamic_files| to 2 the easytags plug-in will
 automatically create project specific tags based on the first name in the
-'tags' option. This disables the global tags file and file type specific tags
-files.
+'tags' option. In this mode the the global tags file or file type specific
+tags files are only used for directories where you don't have write
+permissions.
 
 The |'tags'| option is reevaluated each time the plug-in runs, so which tags
 file is selected can differ depending on the buffer and working directory.