Add feature to remove certain notification

Hi Sueyoshi-san,

Thank you for your awesome extension. I really love it!
If possible, I'd be grateful if you could add the following feature so that I can manually remove notifications from the popup list.

![remove history](https://cloud.githubusercontent.com/assets/6647396/4717463/3c45ca96-5915-11e4-92ba-ece8a0518aae.png)
I thought about it.
`notification_id` is so **long** and I think it's not a good choice for storage and comparison.
what about this
```javascript
project_event._id = util.calcHash(notification_id);
```