Promises are in core, async / await is almost here, let's prepare!

• convert callback APIs into awaitable functions (promisify);
• await multiple tasks with ability to limit concurrecny;
• map with async functions.

Though, there are tests and I am reasonably sure stuff works, I wouldn't put this into production without a review. Things are pretty raw and this is mostly trying out new shiny toys, so approach is "make it work first, make it fast/stable later".

Also, there is nothing here you can't achieve with slightly longer code using Promise.all and some array methods (except limit concurrency). So I don't really know how useful this is yet :)

There are a bunch of examples, run them with --harmony flag.

TODOs:

• filter, reject;
• probably more stuff?
• support for iterables (Set, Map, etc.);
• benchmarks (lol);
• docs.