[SPARK-1273] MLlib bug fixes, improvements, and doc updates for v0.9.1

Cherry-picked a few MLlib commits that are bug fixes, optimization, or doc updates for the v0.9.1 release.

JIRA: https://spark-project.atlassian.net/browse/SPARK-1273

Author: Xiangrui Meng <meng@databricks.com>
Author: Sean Owen <sowen@cloudera.com>
Author: Andrew Tulloch <andrew@tullo.ch>
Author: Chen Chao <crazyjvm@gmail.com>

Closes #175 from mengxr/branch-0.9 and squashes the following commits:

d8928ea [Xiangrui Meng] add Apache header to LocalSparkContext
a66d386 [Xiangrui Meng] Merge remote-tracking branch 'apache/branch-0.9' into branch-0.9
a899894 [Xiangrui Meng] [SPARK-1237, 1238] Improve the computation of YtY for implicit ALS
46fe493 [Xiangrui Meng] [SPARK-1260]: faster construction of features with intercept
6340a18 [Sean Owen] MLLIB-22. Support negative implicit input in ALS
f27441a [Chen Chao] MLLIB-24:  url of "Collaborative Filtering for Implicit Feedback Datasets" in ALS is invalid now
a26ac90 [Sean Owen] Merge pull request #460 from srowen/RandomInitialALSVectors
0564985 [Andrew Tulloch] Fixed import order
2512e67 [Andrew Tulloch] LocalSparkContext for MLlib