Record login timestamp per user. Required for new user managament.

This PR  cleans up the tryRememberLogin method first, moves the core part to the user session and also adds tests for it. This is  necessary, because we need to record the timestamp as well, when a magic cookie (remember checkbox active) is used. The \OC\User\User object gets method to update and return the login timestamp. Updating the timestamp is triggered by postLogin and postRememberedLogin (new!) events. I tested it for login via web interface, remember cookie and desktop client.

There is also a command line tool to get the last login of a specific user. Example:
```
$ ./occ user:lastseen master
master`s last login: 23.05.2014 08:48
```

This PR is required for new user management in #7151.

Please test/review @LukasReschke @icewind1991 @DeepDiver1975