[SPARK-8308] [MLLIB] add missing save load for python example

jira: https://issues.apache.org/jira/browse/SPARK-8308

1. add some missing save/load in python examples. , LogisticRegression, LinearRegression and NaiveBayes
2. tune down iterations for MatrixFactorization, since current number will trigger StackOverflow for default java configuration (>1M)

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #6760 from hhbyyh/docUpdate and squashes the following commits:

9bd3383 [Yuhao Yang] update scala example
8a44692 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into docUpdate
077cbb8 [Yuhao Yang] Merge remote-tracking branch 'upstream/master' into docUpdate
3e948dc [Yuhao Yang] add missing save load for python example