Add exact type checker from react native

[jamiebuilds]

I adapted createStrictShapeTypeChecker from react native into a PropTypes.exact checker.

This operates exactly the same way as PropTypes.shape except it will warn on additional keys:

check(PropTypes.shape({}), { foo: 1 }); // Okay.
check(PropTypes.exact({}), { foo: 1 }); // Warning!

This is a runtime version of Flow's exact object types:

type shape = { ... };
type exact = {| ... |};

[aweary]

Verified tests pass locally. LGTM.

[gaearon]

I reverted in #80 because the review missed a few breakages related to how this repo is setup (sorry they're not obvious 😞 ).

Happy to take it back once they're addressed!