PIG-4327: Schema of map with value that has an alias can't be parsed again

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1639742 13f79535-47bb-0310-9956-ffa450edef68
PIG-4331: update README, '-x' option in usage to include tez

git-svn-id: https://svn.apache.org/repos/asf/pig/trunk@1639805 13f79535-47bb-0310-9956-ffa450edef68