Add class to retrieve traffic statistics

As a program evaluator, I want to be be able to retrieve
Wikipedia article traffic statistics.

- Add module to retrieve article traffic statistics from the
stats.grok.se service.
- Add unit test class.
Traffic statistics: add method to build stats.grok.se

The stats.grok.se web service can provide traffic statistics,
in JSON format, at an URL constructed using the article title,
the project, the year and the month.

Add method _make_URL() to make such an URL
Add unit test for it