Add 'with' function to attach custom props to the event

This PR adds a 'with' function to the handler fn so that you can set custom properties in the event before dispatching it into the `Bacon.Bus`. A typical use case for this is when you have a component that is simple enough to have the children rendered inline, and we have a handler in each particular child:


```js
/** @jsx React.DOM */
...
componentWillMount: function() {
  this.eventStream('itemClicks').
    doAction('.preventDefault').
    map('.itemId'). // I have access to the item id that was clicked
    ...
},
render: function() {
  var items = this.props.items.map(this.renderItem);
  return <ul>{items}</ul>
},

renderItem: function(item) {
  return <li><a onClick={ this.itemClicks.with({ itemId: item.id }) }></a></li>
}
```

I think this is a handy feature, and I've already needed something like this when using this lib, which is awesome by the way.