Allow map to return objects

[megawac]

Map currently will attempt to assign keys to arrays and because of a bug in nodeunit this passes unit tests. I think map should either always return an array (ala lodash/underscore) for objects or do as this PR does

/cc @aearly

[aearly]

I'm not sure what the "correct" thing to do here is. The way it works now is just a happy accident of using eachOf internally. I agree that map should always return arrays. Lodash returns an array when you pass an array to _.map, and has a _.mapValues function that returns an object. We could add async.mapValues (and mapValuesSeries and mapValuesLimit), but as you can see, it gets out of hand.

Once we support iterators, one thing that will be really useful is being able to have map callback with another iterator -- it will enable doing some cool stuff with lazy sequences. But perhaps we need another method for a mapping operation that keeps the type of the iterable you pass in.

[aearly]

Another thought -- Haskell and other FP languages have the concept of an fmap function -- a function that takes a functor (basically anything that is mappable) and returns a new instance of that functor with the mapping function applied to each element. Perhaps we could call the new method async.fmap? (and fmapSeries and fmapLimit)

[aearly]

Going to merge this for now, we can have map return an array later.