Hovering a Timeago-String should show the Timestamp

Fixes https://github.com/diaspora/diaspora/issues/3149

Inbox:
![diaspora-bug-ui-timestamp](https://f.cloud.github.com/assets/1857707/100074/ef01cf0c-67e7-11e2-8d8f-98c19513c0a8.png)

I can't easily add a timestamp on the inbox with the currenly way it's working, neither tooltips (as below). But I fixed the timeago thing.

Stream:
![diaspora-bug-ui-timestamp1](https://f.cloud.github.com/assets/1857707/100075/fa1689e6-67e7-11e2-8ea5-e55fa2671562.png)

I added a tooltip format to the timestamp to keep it unified with the close and hide buttons. I think the timeago underline should be removed.