Fix timeago for notifications, add 'no notifications yet' message

![no notifications yet](https://cloud.githubusercontent.com/assets/3798614/5157613/b779053c-7316-11e4-9d4f-4d51df7c5322.png)

We have the same code for the notifications badge (see this PR), as well as the conversations: https://github.com/diaspora/diaspora/blob/develop/app/views/conversations/show.js.erb#L13 https://github.com/diaspora/diaspora/blob/develop/app/assets/javascripts/app/views/conversations_view.js#L21

I thought about moving that code to a helper function but the functions in /app/assets/javascripts/app/helpers/ look completely different because they don't directly change stuff in the views. Any ideas?