Update ChangeLog with the last three bug fixes/ehancements.

Also, in a lame attempt att giving credit for last commit where I
failed at doing that:

  4920ff44 is a patch from Fabian Mauchle.

ChangeLog

+2013-      1.6.3
+	Enhancements:
+	- Threads are allocated with a 32 KB stack rather than what
+	happens to be the default. Patch by Fabian Mauchle.
+	- On systems with mallopt(3), freed memory is returned to the
+	system more aggressively. Patch by Fabian Mauchle.
+
+	Bug fixes:
+	- The duplication cache is purged properly. Patch by Fabian
+	Mauchle.
+
 2012-10-25 1.6.2
 	Bug fixes (security):
 	- Fix the issue with verification of clients when using multiple