Select action type

### Abstract

The more the registered actions, the more time the dispatcher spent time to choice matched callback.
So this p-r enables to give actionType to register action optionally.

### Usage

```javascript
var dispatcher = require("flux").Dispatcher;

dispatcher.register("some-event", function(payload){
  // without checking payload.actionType
})

var payload = {
  actionType: "some-event"
}
dispatcher.dispatch(payload); //=> fire registered callback with "some-event"
```