Use string timestamp for docker events -since

```sudo docker events -since 1378216169```

is much harder to get right than 

```sudo docker events -since '2013-09-03 15:49:29 +0200 CEST'```

the date format that works is the one that is printed out by ```docker events``` - not as awesome as being able to parse any kind of date, but much more possible for most users :)

Closes #2328