[SQL] Simple DSL support for Scala UDF

This feature is based on an offline discussion with @mengxr, hopefully can be useful for the new MLlib pipeline API.

For the following test snippet

```scala
case class KeyValue(key: Int, value: String)
val testData = sc.parallelize(1 to 10).map(i => KeyValue(i, i.toString)).toSchemaRDD
def foo(a: Int, b: String) => a.toString + b
```

the newly introduced DSL enables the following syntax

```scala
import org.apache.spark.sql.catalyst.dsl._
testData.select(Star(None), foo.call('key, 'value))
```

which is equivalent to

```scala
testData.registerTempTable("testData")
sqlContext.registerFunction("foo", foo)
sql("SELECT *, foo(key, value) FROM testData")
```