Fix report activities (nothing was returned without types)

* Not specifying `types` parameter returns nothing, which was not covered by the testsuite. Tricky to test though, since returning no elements might be a valid outcome anyway (no activities for defined time range):

```
{u'activities': [], u'status': u'success'}
```

* Reasonable defaults. I'm using the parameters that `Solve360` is using by default in their web interface when checking the XHR calls (principle of least surprise in the API).

* Put a docstring in the method, useful while figuring out the API for the first time via ipython. Easy and quick programming reference.