Add Ko getter method to Board

[artasparks]

We should add a Ko helper to the Board struct; currently ko is private: https://github.com/otrego/clamshell/blob/master/core/board/board.go#L17

Background:

• For more about ko in go, see: https://en.wikipedia.org/wiki/Ko_fight
• For more about unit testing: https://golang.org/pkg/testing/

Requirements:

• Should have a unit test to get the Ko
• Should have a unit test that exercises Ko-logic (in a black box fashion via PlaceMove).