changes to CHANGES.txt to add PIG-3512 and move PIG-3325 change log

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1534833 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd