Clarify value/reason propagation

Specifically, add clause to clarify when onFulfilled/onRejected are
not functions, promise1's value/reason must be propagated.

Thanks @getify. See twitter convo:
https://twitter.com/getify/status/463297602084032513
Merge pull request #164 from briancavalier/clarify-non-function-propagation

Clarify value/reason propagation