[SPARK-6117] [SQL] add describe function to DataFrame for summary statis...

Please review my solution for SPARK-6117

Author: azagrebin <azagrebin@gmail.com>

Closes #5073 from azagrebin/SPARK-6117 and squashes the following commits:

f9056ac [azagrebin] [SPARK-6117] [SQL] create one aggregation and split it locally into resulting DF, colocate test data with test case
ddb3950 [azagrebin] [SPARK-6117] [SQL] simplify implementation, add test for DF without numeric columns
9daf31e [azagrebin] [SPARK-6117] [SQL] add describe function to DataFrame for summary statistics