Issue192 - dynamic import of user defined modules

Users can add new attribute in [GLOBAL] section to defined metric, like:

```
[GLOBAL]
user_defined_metrics=MYMETRIC1:MyMetric1Class:/path/to/my_metric1.py MYMETRIC2:MyMetric2Class:/path/to/my_metric2.py
```

And use them in the config to analyze the new metrics.