Unions Types and User Defined Type Guard

[basarat]

microsoft/TypeScript#1007

http://stackoverflow.com/a/34193842/390330

[HerringtonDarkholme]

Looking forward to TypeScript book updated!

I think this pull-request worth mentioning.
It refines union/intersection type inference. And it enables user-defined nullable types, which is a pain point not resolved in TS's type system but in others(alas, flow-type).

[basarat]

@HerringtonDarkholme Noted for inclusion 🌹