IMPALA-727 IMPALA-791: Support String type in Hive UDFs.

Change-Id: I5ecd58054a71edd2c43f8d31c326be87b362489a
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1509
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins