extend VASTParser to emit 'VAST-error' event

VASTParser now has an instance of EventEmitter. It exposes 
```
on: (eventName, cb) ->
once: (eventName, cb) ->
```

So you can do

```
VASTParser.on 'VAST-error', (error) -> console.log 'Vast-error occured', error.ERRORCODE
```

what do you think?