fix stats of closed bugs (closes #52)

closed bugs are only the ones without type=0 (not open)

Result:
```
<tr><td>Oct 2013</td><td>3280</td><td>7061</td></tr>
<tr><td>Sep 2013</td><td>3520</td><td>4761</td></tr>
<tr><td>Aug 2013</td><td>2874</td><td>4075</td></tr>
<tr><td>Jul 2013</td><td>4006</td><td>4026</td></tr>
```

Pulling it is not that important.

Regards
Werner