[SQL] Improve SparkSQL Aggregates

* Add native min/max (was using hive before).
* Handle nulls correctly in Avg and Sum.

Author: Michael Armbrust <michael@databricks.com>

Closes #683 from marmbrus/aggFixes and squashes the following commits:

64fe30b [Michael Armbrust] Improve SparkSQL Aggregates * Add native min/max (was using hive before). * Handle nulls correctly in Avg and Sum.

(cherry picked from commit 19c8fb02bc2c2f76c3c45bfff4b8d093be9d7c66)
Signed-off-by: Reynold Xin <rxin@apache.org>