Added support for underscores-as-field-traversal

Summary:
Ref T9270

Merged from [livingsocial@e62d859](https://github.com/livingsocial/Hive-JSON-Serde/commit/e62d859a407f1cf95cf31e24e608b38e0e83d0eb). Summary of that commit:

For example, if I have a JSON record that looks like this:
```
{
  "address": {
     "city": "Baltimore"
  },
  "phone_number": "301-555-2525"
}
```

I can define Hive columns that look this:

```
CREATE TABLE json_test (
           address_city string,
           phone_number string,
) ROW FORMAT SERDE 'org.openx.data.jsonserde.JsonSerDe'
STORED AS TEXTFILE;
```

and a `SELECT` query would return the following:

```
hive> SELECT * FROM json_test;
OK
Baltimore 301-555-2525
Time taken: 0.432 seconds
```

Test Plan: `mvn package`, + @JamesGreenhill

Reviewers: jeff, JamesGreenhill

Reviewed By: JamesGreenhill

Subscribers: ernestwaynewong, JamesGreenhill

Projects: #data

Maniphest Tasks: T9270

Differential Revision: http://phabricator.local.disqus.net/D12995