SPARK-2186: Spark SQL DSL support for simple aggregations such as SUM and AVG

**Description** This patch enables using the `.select()` function in SchemaRDD with functions such as `Sum`, `Count` and other.
**Testing** Unit tests added.
There's no implicitness going on in here, since the user needs to explicitly call both `sum` and `disctinct`. I have no problem changing it, though.

Fixed.