Use numpy directly for matrix multiply.

Using matrix multiply to compute XtX and XtY yields a 5-20x speedup depending on problem size.

For example - the following takes 19s locally after this change vs. 5m21s before the change. (16x speedup).
bin/pyspark examples/src/main/python/als.py local[8] 1000 1000 50 10 10

Author: Evan Sparks <evan.sparks@gmail.com>

Closes #687 from etrain/patch-1 and squashes the following commits:

e094dbc [Evan Sparks] Touching only diaganols on update.
d1ab9b6 [Evan Sparks] Use numpy directly for matrix multiply.

(cherry picked from commit 6ed7e2cd01955adfbb3960e2986b6d19eaee8717)
Signed-off-by: Reynold Xin <rxin@apache.org>