added PromiseSequence that waits asynchronously for each Promise item

[FrozenCow]

As described in the previous issue, this will allow people to create a Sequence of promises and let Lazy wait for each of them to be resolved. This allows functionality like mapping a sequence of URLs to a sequence of HTML-fetching-promises and, with mapThen, resolve those promises to a sequence of HTML.

[FrozenCow]

It might be that I've overlooked the obvious and didn't know this was already possible with the existing functions in Lazy.js. If so, let me know how to lazily wait for a sequence of promises.

Also, please check whether you agree with the changes made to altered async_sequence_spec. I wasn't too sure how to solve this cleanly and would like feedback on this.