Checking in changes prior to tagging of version 0.51.

Changelog diff is:

diff --git a/Changes b/Changes
index 4f6d2ea..b7d878c 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,7 @@
 Revision history for Perl extension CGI::Header.

+0.51 (1.00-RC4) May 13th, 2013
+
   - Add CGI::Header::Standalone that doesn't depend on CGI.pm's
     header() method
   - Add clone() method to CGI::Header