[PATCH] note option

This patch add the "note" option to squid.conf

The patch originally posted and discussed in squid-dev under the
"[PATCH] meta_header option" thread:
   http://www.squid-cache.org/mail-archive/squid-dev/201209/0178.html
   http://www.squid-cache.org/mail-archive/squid-dev/201210/0127.html

More informations and documentation exists in patch preamble.

Changes over the original patch:
 - Rename "meta_header" configuration variable to "note"
 - Rename "%meta_header" formating code to %note
 - Rename:
    MetaHeader class to Note
    MetaHeaders class to Notes
 - Create the NotePairs class which is based on HttpHeader class
 - Move new classes in Notes.h and Notes.cc files

Regards,
    Christos