Updates distance matrix tool to allow non ascii characters in id fiel…

…ds. This is based on a workaround (from http://docs.python.org/library/csv.html), as the default Python csv function(s) do not support nonascii characters. Fixes #2496

git-svn-id: http://svn.osgeo.org/qgis/trunk@13019 c8812cc2-4d05-0410-92ff-de0c093fc19c