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)
`current_time` is not unique value
If receive multiple events at the same time, `_id` is duplicated
I recommend `notification_id` immediately above