Add metrics to notification

This adresses #476. `Notification.BigTextStyle` was only added in API level 16. I think the notification text can only be one line though, which is a bit too short.

@garvankeeley 
> Keep in mind this code recreates the notification every time the message changes, which has the effect in the UI of the row deleting (with animation) and creating a new row, so the update should be rate limited.

There is no animation during an update on Android 5. I did not investiage yet how other devices (>= API 16) do this though.

Pulled down notification on Android 5 lock screen:
![notification](https://cloud.githubusercontent.com/assets/3845150/5151651/b1323116-71e1-11e4-9531-26a3ef50d0a9.png)