MapStream

Refactored out of event-stream

##map (asyncFunction[, options])

Create a through stream from an asyncronous function.

`` js
var map = require('map-stream')

map(function (data, callback) {
//transform data
// ...
callback(null, data)
})

`

Each map MUST call the callback. It may callback with data, with an error or with no arguments,

* callback() drop this data.
this makes the map work like filter,
note:callback(null,null) is not the same, and will emit null

* callback(null, newData) turn data into newData

* callback(error) emit an error for this item.

>Note: if a callback is not called, map will think that it is still being processed,
>every call must be answered or the stream will not know when to end.
>
>Also, if the callback is called more than once, every call but the first will be ignored.

##Options

* failures - boolean continue mapping even if error occured. On error map-stream will emit failure event. (default: false`)