Write Virginia Job Postings generator

[thcrock]

Similar to #160 we would like more job posting generators with fewer dependencies. A cheap way of getting a bunch of job postings with the simple dependency of an internet connection is to hook up to the open data Virginia job postings:

http://opendata.cs.vt.edu/dataset/ab0abac3-2293-4c9d-8d80-22d450254389

They are already in common schema format, so the class pretty much just has to download them and stream them, similar to the example: https://github.com/workforce-data-initiative/skills-ml/blob/master/examples/NounPhraseSkillExtraction.py.

Bonus points: allow filtering by month