Add `sampleBy` to DataFrame. rxin

Author: Xiangrui Meng <meng@databricks.com>

Closes #6769 from mengxr/SPARK-7157 and squashes the following commits:

991f26f [Xiangrui Meng] fix seed
4a14834 [Xiangrui Meng] move sampleBy to stat
832f7cc [Xiangrui Meng] add sampleBy to DataFrame