Summary:
Adds two methods ```AddChild``` and ```RemoveChild``` to be able to add/remove children without the need to know there position.
Closes https://github.com/facebook/yoga/pull/513
Reviewed By: emilsjolander
Differential Revision: D4914242
Pulled By: splhack
fbshipit-source-id: df187ba71a1bae0b3b0649624f3c29401d5f8804