second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
Move bare imports above all the `from .. import` statements. It's cleaner to look at. Also, there is a convention we follow to order the imports based on their *scope*, i.e. standard library first, then third-party libraries, then local packages and modules. So in this case it would look like this:

```python
import time
import json
import logging
import vcf
from cStringIO import StringIO
from django.db.models import Q
from avocado.export._base import BaseExporter
from varify.variants.models import Variant
from varify.samples.models import Result
```