Early WIP: Web Notifications

I started first iteration for html notifications feature requested in https://github.com/travis-ci/travis-ci/issues/1109. No time to code review yet. Just tech demo for http://www.w3.org/TR/notifications/. I will be glad to finish this. I just need to know if this is wanted and if someone can help me with programming-less problems.

## Screenshot from my linux Chrome:

![travis-notify](https://cloud.githubusercontent.com/assets/193936/2964813/c6d9e920-daea-11e3-9e92-4d277a04d0f0.png)


# Problems: 

- [ ] commit message is not available everytime
- [ ] UI for notification control (enable/disable) is not done
- [ ] design icons for passed, failed, errored and canceled notifications (I did some ugly ones for testing).
- [ ] decide what will be best copy for those notifications, currently it is (title, body):

```
"repo.slug" #"build.number" "build.state"
"commit.message"
```