second attempt to add the VCF exporter

The VCF exporter finally generates output that can be used in R.
I plan on doing so, eventually.  I've seen some issues within the past 6 months pertaining to how the writer API should be enhanced, so I should try using the PyVCF HEAD before writing any issues myself.  Unfortunately, it's been awhile since the last stable release, and I doubt a new one is coming anytime soon, so I think we'll have to live with a workaround for awhile.