[SPARK-5128][MLLib] Add common used log1pExp API in MLUtils

When `x` is positive and large, computing `math.log(1 + math.exp(x))` will lead to arithmetic
overflow. This will happen when `x > 709.78` which is not a very large number.
It can be addressed by rewriting the formula into `x + math.log1p(math.exp(-x))` when `x > 0`.

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #3915 from dbtsai/mathutil and squashes the following commits:

bec6a84 [DB Tsai] remove empty line
3239541 [DB Tsai] revert part of patch into another PR
23144f3 [DB Tsai] doc
49f3658 [DB Tsai] temp
6c29ed3 [DB Tsai] formating
f8447f9 [DB Tsai] address another overflow issue in gradientMultiplier in LOR gradient code
64eefd0 [DB Tsai] first commit