[SPARK-8246] [SQL] Implement get_json_object

This is based on #7485 , thanks to NathanHowell

Tests were copied from Hive, but do not seem to be super comprehensive. I've generally replicated Hive's unusual behavior rather than following a JSONPath reference, except for one case (as noted in the comments). I don't know if there is a way of fully replicating Hive's behavior without a slower TreeNode implementation, so I've erred on the side of performance instead.

Author: Davies Liu <davies@databricks.com>
Author: Yin Huai <yhuai@databricks.com>
Author: Nathan Howell <nhowell@godaddy.com>

Closes #7901 from davies/get_json_object and squashes the following commits:

3ace9b9 [Davies Liu] Merge branch 'get_json_object' of github.com:davies/spark into get_json_object
98766fc [Davies Liu] Merge branch 'master' of github.com:apache/spark into get_json_object
a7dc6d0 [Davies Liu] Update JsonExpressionsSuite.scala
c818519 [Yin Huai] new results.
18ce26b [Davies Liu] fix tests
6ac29fb [Yin Huai] Golden files.
25eebef [Davies Liu] use HiveQuerySuite
e0ac6ec [Yin Huai] Golden answer files.
940c060 [Davies Liu] tweat code style
44084c5 [Davies Liu] Merge branch 'master' of github.com:apache/spark into get_json_object
9192d09 [Nathan Howell] Match Hive’s behavior for unwrapping arrays of one element
8dab647 [Nathan Howell] [SPARK-8246] [SQL] Implement get_json_object