[SPARK-2594][SQL] Support CACHE TABLE <name> AS SELECT ...

This feature allows user to add cache table from the select query.
Example : ```CACHE TABLE testCacheTable AS SELECT * FROM TEST_TABLE```
Spark takes this type of SQL as command and it does lazy caching just like ```SQLContext.cacheTable```, ```CACHE TABLE <name>``` does.
It can be executed from both SQLContext and HiveContext.

Recreated the pull request after rebasing with master.And fixed all the comments raised in previous pull requests.
https://github.com/apache/spark/pull/2381
https://github.com/apache/spark/pull/2390

Author : ravipesala ravindra.pesala@huawei.com

Thank you for your comment. It is a good idea to import ```sqlContext._```. But we can simplify as below code if we import it. Please comment on it.
```
import sqlContext._
plan.registerTempTable(tableName)
cacheTable(tableName) 
```