Checking in changes prior to tagging of version 0.18.

Changelog diff is:

diff --git a/Changes b/Changes
index 3fe4882..2bcc004 100644
--- a/Changes
+++ b/Changes
@@ -1,5 +1,9 @@
 Revision history for Algorithm::HyperLogLog

+0.18    Sun Jun 16 23:50:00 2013
+        - Update dependency
+        - Change document format
+
 0.17    Fri Jun 14 02:02:00 2013
         - Fixed warning in perl-5.18