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.

Author: Ximo Guanter Gonzalbez <ximo@tid.es>

Closes #1211 from edrevo/add-expression-support-in-select and squashes the following commits:

fe4a1e1 [Ximo Guanter Gonzalbez] Extend SQL DSL to functions
e1d344a [Ximo Guanter Gonzalbez] SPARK-2186: Spark SQL DSL support for simple aggregations such as SUM and AVG