[SPARK-2993] [MLLib] colStats (wrapper around MultivariateStatisticalSummary) in Statistics

For both Scala and Python.

The ser/de util functions were moved out of `PythonMLLibAPI` and into their own object to avoid creating the `PythonMLLibAPI` object inside of `MultivariateStatisticalSummarySerialized`, which is then referenced inside of a method in `PythonMLLibAPI`.

`MultivariateStatisticalSummarySerialized` was created to serialize the `Vector` fields in `MultivariateStatisticalSummary`.

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1911 from dorx/colStats and squashes the following commits:

77b9924 [Doris Xin] developerAPI tag
de9cbbe [Doris Xin] reviewer comments and moved more ser/de
459faba [Doris Xin] colStats in Statistics for both Scala and Python

(cherry picked from commit fe4735958e62b1b32a01960503876000f3d2e520)
Signed-off-by: Xiangrui Meng <meng@databricks.com>