Pathname Objects

Web#files_path and Web#blatex_pngs_path now return Pathname objects.
Based on JHerdman's
5d1e8f420b
but requires several other changes to the code (which assumed a string).

Also, test for itex2MML 1.3.10 (you should update that too).

app/models/wiki_file.rb

@@ -46,7 +46,7 @@ class WikiFile < ActiveRecord::Base
   end
   
   def content_path
-    web.files_path + '/' + file_name
+    web.files_path.join(file_name)
   end
   
   def write_content_to_file