Add add.raw.json.column SerDe property

[wonder-mice]

Added SerDe property "add.raw.json.column" can be used to include
entire raw json string into the row as separate field.
For example.

CREATE TABLE test_01 (
type string
)
ROW FORMAT SERDE "org.openx.data.jsonserde.JsonSerDe"
WITH SERDEPROPERTIES ("add.raw.json.column" = "raw_json");

will create table with additional "raw_json" field. That field will
contain raw json string. That can be usefull when json structure
is not uniform and later processing with UDTF is desired.

[rcongiu]

This is a good idea, however, could you also add unit tests for it ? If not it's fine, i can do it, but it would speed up the merge of the pull request.