GSoC python variant update 7

Post: http://arklenna.tumblr.com/post/26812132902/

Synopsis:
This week, I wrote a script for PyVCF that can filter a file by sample
as it's being parsed. It's currently named `vcf_sample_filter.py`.
It's designed to be functional from the command line, the Python
interpreter, or as a module.

Next up: come up with a generic-via-extensibility representation of a
variant. I'm working through some examples and should have a basic
outline soon.

Lenna