[SPARK-5317]Set BoostingStrategy.defaultParams With Enumeration...

[SPARK-5317]Set BoostingStrategy.defaultParams With Enumeration Algo.Classification or Algo.Regression

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-5317
When setting the BoostingStrategy.defaultParams("Classification"), It's more straightforward to set it with the Enumeration Algo.Classification, just like BoostingStragety.defaultParams(Algo.Classification).
I overload the method BoostingStragety.defaultParams().

Author: Basin <jpsachilles@gmail.com>

Closes #4103 from Peishen-Jia/stragetyAlgo and squashes the following commits:

87bab1c [Basin] Docs and Code documentations updated.
3b72875 [Basin] defaultParams(algoStr: String) call defaultParams(algo: Algo).
7c1e6ee [Basin] Doc of Java updated. algo -> algoStr instead.
d5c8a2e [Basin] Merge branch 'stragetyAlgo' of github.com:Peishen-Jia/spark into stragetyAlgo
65f96ce [Basin] mllib-ensembles doc modified.
e04a5aa [Basin] boostingstrategy.defaultParam string algo to enumeration.
68cf544 [Basin] mllib-ensembles doc modified.
a4aea51 [Basin] boostingstrategy.defaultParam string algo to enumeration.