Checking in changes prior to tagging of version 0.18.

Changelog diff is:

diff --git a/Changes b/Changes
index 3b79486..d2fd4e6 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,7 @@
 Revision history for Perl extension CGI::Header.

+0.18  Feb 9th, 2013
+
   - You can pass a media type to new() in the following situation:

       my $h = CGI::Header->new('text/plain');