[SPARK-4559][SQL] Adding support for ucase and lcase

Adding support for ucase and lcase in spark sql
These 4 lines can be merged:

```scala
     | (UPPER | UCASE) ~ "(" ~> expression <~ ")" ^^ { case exp => Upper(exp) }
     | (LOWER | LCASE) ~ "(" ~> expression <~ ")" ^^ { case exp => Lower(exp) }
```