Minor fix: made "EXPLAIN" output to play well with JDBC output format

Fixed the broken JDBC output. Test from Shark `beeline`:

```
beeline> !connect jdbc:hive2://localhost:10000/
scan complete in 2ms
Connecting to jdbc:hive2://localhost:10000/
Enter username for jdbc:hive2://localhost:10000/: lian
Enter password for jdbc:hive2://localhost:10000/:
Connected to: Hive (version 0.12.0)
Driver: Hive (version 0.12.0)
Transaction isolation: TRANSACTION_REPEATABLE_READ
0: jdbc:hive2://localhost:10000/>
0: jdbc:hive2://localhost:10000/> explain select * from src;
+-------------------------------------------------------------------------------+
|                                     plan                                      |
+-------------------------------------------------------------------------------+
| ExplainCommand [plan#2:0]                                                     |
|  HiveTableScan [key#0,value#1], (MetastoreRelation default, src, None), None  |
+-------------------------------------------------------------------------------+
2 rows selected (1.386 seconds)
```

Before this change, the output looked something like this:

```
+-------------------------------------------------------------------------------+
|                                     plan                                      |
+-------------------------------------------------------------------------------+
| ExplainCommand [plan#2:0]
 HiveTableScan [key#0,value#1], (MetastoreRelation default, src, None), None  |
+-------------------------------------------------------------------------------+
```

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1097 from liancheng/multiLineExplain and squashes the following commits:

eb37967 [Cheng Lian] Made output of "EXPLAIN" play well with JDBC output format