Mail charset error support

- Try multiple charset when no charset is specified by "Content-Type" field :
begins with sys.defaultencoding utf8
then "iso-8859-1", config file encoding and finally encoding found in previous
fields "Subject" or "From".

darcs-hash:20060201110843-86b55-7fd556a8ffadd9a5fed2b3317b17bfcca9d1dc58.gz

tests/jmc-test.xml

@@ -15,4 +15,5 @@
   <storage>SQLite</storage>
   <spooldir>.</spooldir>
   <check_interval>5</check_interval> <!-- in minutes -->
+  <mail_default_encoding>iso-8859-15</mail_default_encoding>
 </config>