Fixes Issue #184.
Follows the same pattern as map and mapSeries, almost a direct copypasta of those (they both reuse the corresponding forEach functions)
Unfortunately still lacks tests because I suck at that kind of thing.
I was really excited when I saw this request, because I wanted a mapLimit function. I tried pulling your patch, and it doesn't work because the functions have different arity :(.
I submitted my own at #202. Hopefully it's useful if you wanted mapLimit functionality.
Okay, closing this one for #202. Thanks, @calvinfo!