[SPARK-7612] [MLLIB] update NB training to use mllib's BLAS

This is similar to the changes to k-means, which gives us better control on the performance. dbtsai

Author: Xiangrui Meng <meng@databricks.com>

Closes #6128 from mengxr/SPARK-7612 and squashes the following commits:

b5c24c5 [Xiangrui Meng] merge master
a90e3ec [Xiangrui Meng] update NB training to use mllib's BLAS