Update DateTime.d.ts

[jinyi0521]

When you want to use step, it has bug about min and max values

Description

I wanted to use only step option, but I have to set up min and max values. And it has bug that it doesnt work as I intended to. So I changed min and max as a optional typing. It works.

Motivation and Context

1. Someone like me want to use only step, but this code doesnt allow us to use it.
2. Yes it does

Checklist

[x ] I have not included any built dist files (us maintainers do that prior to a new release)
[x ] I have added tests covering my changes
[ x] All new and existing tests pass
[x ] My changes required the documentation to be updated
  [x ] I have updated the documentation accordingly
  [ x] I have updated the TypeScript 1.8 type definitions accordingly
  [ x] I have updated the TypeScript 2.0+ type definitions accordingly