ConsoleDataWriter displays errors summary. Resolves #1770

Now output format looks like:
```
================================================================================
2014-04-23 11:45:03                                           1s elapsed
---- My scenario ---------------------------------------------------------------
[##########################################################################]100%
          waiting: 0      / running: 0      / done:1
---- Requests ------------------------------------------------------------------
> Global                                                   (OK=41     KO=20    )
> request_1                                                (OK=20     KO=0     )
> request_2                                                (OK=20     KO=0     )
> request_3                                                (OK=1      KO=0     )
> request_4                                                (OK=0      KO=20    )
---- Errors --------------------------------------------------------------------
> status.in(200,304,201,202,203,204,205,206,207,208,209... (count=20    (100 %))
================================================================================
```

is it fine?

What about "delta since last display" that was mentioned in the issue description? How exactly this should be displayed?