Adding Bacon.fromMultiCallback

Bacon.fromMultiCallback creates an EventStream from a function that accepts a callback. The function may call its callback multiple times (in the same tick of the event loop or across multiple ticks), with each call resulting in a new event in the stream. 

For example:

```js
var bacons = Bacon.fromMultiCallback(function(callback) {
  setInterval(function() {
    callback("Bacon!")
  }, 1000)
})

bacons.take(3).onValue(console.log) // Bacon! Bacon! Bacon!
```