second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
The exporter generates a conventional VCF file.  It can work with either the iterator passed from Serrano, or a client that sends a POST request using JSON.  While this is designed for use with a particular client, anything that sends the appropriate HTTP request and can invoke the exporter.
The designated client just spits out the VCF file verbatim.