[SPARK-5207] [MLLIB] StandardScalerModel mean and variance re-use

This seems complete, the duplication of tests for provided means/variances might be overkill, would appreciate some feedback.

Author: Octavian Geagla <ogeagla@gmail.com>

Closes #4140 from ogeagla/SPARK-5207 and squashes the following commits:

fa64dfa [Octavian Geagla] [SPARK-5207] [MLLIB] [WIP] change StandardScalerModel to take stddev instead of variance
9078fe0 [Octavian Geagla] [SPARK-5207] [MLLIB] [WIP] Incorporate code review feedback: change arg ordering, add dev api annotations, do better null checking, add another test and some doc for this.
997d2e0 [Octavian Geagla] [SPARK-5207] [MLLIB] [WIP] make withMean and withStd public, add constructor which uses defaults, un-refactor test class
64408a4 [Octavian Geagla] [SPARK-5207] [MLLIB] [WIP] change StandardScalerModel contructor to not be private to mllib, added tests for newly-exposed functionality