adding initial code for StratifiedShuffleSplit - #243

There is a test failure due to the doc processor cannot handle certain type. Could you please take a look at it? Let me know if it's too hard.

sure will do

Also, could you make sure these things?

1. Export StratifiedShuffleSplit in index.ts (check model_selection/index.ts) so the users can actually use it
2. add an integration test
3. Please add unit tests to ensure 100% coverage and all the behaviours for this code
4. You need to add doc strings (check other APIs) so the doc processor can actually pick it up and generate documentation.

Also, could you make sure these things?

1. Export `StratifiedShuffleSplit` in `index.ts` (check model_selection/index.ts) so the users can actually use it

2. add an integration test

3. Please add unit tests to ensure 100% coverage and all the behaviours for this code

4. You need to add doc strings (check other APIs) so the doc processor can actually pick it up and generate documentation.

fixed

@JasonShin please review this