PIG-3367: Add assert keyword (operator) in pig (aniket486)

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1525045 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk
Merge remote branch 'origin/trunk' into trunk-gd