[SPARK-1968][SQL] SQL/HiveQL command for caching/uncaching tables

JIRA issue: [SPARK-1968](https://issues.apache.org/jira/browse/SPARK-1968)

This PR added support for SQL/HiveQL command for caching/uncaching tables:

```
scala> sql("CACHE TABLE src")
...
res0: org.apache.spark.sql.SchemaRDD =
SchemaRDD[0] at RDD at SchemaRDD.scala:98
== Query Plan ==
CacheCommandPhysical src, true

scala> table("src")
...
res1: org.apache.spark.sql.SchemaRDD =
SchemaRDD[3] at RDD at SchemaRDD.scala:98
== Query Plan ==
InMemoryColumnarTableScan [key#0,value#1], (HiveTableScan [key#0,value#1], (MetastoreRelation default, src, None), None), false

scala> isCached("src")
res2: Boolean = true

scala> sql("CACHE TABLE src")
...
res3: org.apache.spark.sql.SchemaRDD =
SchemaRDD[4] at RDD at SchemaRDD.scala:98
== Query Plan ==
CacheCommandPhysical src, false

scala> table("src")
...
res4: org.apache.spark.sql.SchemaRDD =
SchemaRDD[11] at RDD at SchemaRDD.scala:98
== Query Plan ==
HiveTableScan [key#2,value#3], (MetastoreRelation default, src, None), None

scala> isCached("src")
res5: Boolean = false
```

Things also work for `hql`.

This is going to be updated to some standard command interface in a follow up PR?