Checking in changes prior to tagging of version 0.17.

Changelog diff is:

diff --git a/Changes b/Changes
index fb2a300..8c7d4de 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,8 @@
 Revision history for Perl extension Plack::Middleware::Session

+0.17 Mon Feb 11 15:40:50 PST 2013
+    - Use constant time comparison of HMAC signature to be precautious for timing attacks
+
 0.16 Sun Feb 10 11:41:14 PST 2013
     - Fix minimum version requirement for Test::Fatal