Replace Future#promise() with Promises/A+ compatible Future#then()

[Avaq]

So this might be an interesting topic for conversation. Using the new CachedFuture class I created while working on #34, I was able to create a fully Promises/A+ compliant then method. Essentially turning Fluture into a Promise library! o.0

I think the library might benefit from having this. Particularly because fork is not standardized. Though I'm very eager to hear peoples opinions. Check out the diff in the readme to learn about how I advice the feature to be used.

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[Avaq]

An alternative is to provide then as such:

function(onResolve, onReject){
  return new Promise((res, rej) => this._f(rej, res)).then(onResolve, onReject);
}

And not concern myself with the complexity of Promises. The intended use for then would remain the same. The only difference being that you won't be able to use Future methods on the return value - perhaps even for the best.

[davidchambers]

I'm concerned that this may confuse new users of the library.

[Avaq]

I think it's quite cool that Futures can also be Promises. But that's where I run out of pro's. I think making Futures Promises enables a bunch of bad practices and nothing more. I believe providing a then function which returns a Promise has less pitfalls, but offers nothing more than a magical convenience over .promise(). I'm sure this PR will enjoy its new life as reference material. ;)