second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
I moved it to base and updated the references, will be pushing as soon as the local tests finish. Please remove this and import `QueueTestCase` from its new home in `tests.cases.base` once I update master with the changes. Just add

```python
from ..base import QueueTestCase
```

to your imports.