adding fromMultiCallback to Bacon

Creates an EventStream from a function that accepts a callback. The function can be called multiple times, with each call resulting in a new event in the stream. For example:

foos = Bacon.fromMultiCallback(function(callback) {
  window.setInterval(function() {
    callback('foo');
  }, 500);
});

foos.take(3).log() // foo foo foo
Updating README with fromMultiCallback