Filter out issues_closed, attribution is wrong at the moment

Github API doesn't allow to distinguish between closer and author,
see https://github.com/chillu/github-dashing/issues/12
API Removed big query support

The project lead (Ilya Grigorik) acknowledged the limitations as structural,
so we can't easily work around them. Quote:

"In retrospect, I wish I did partition the dataset into monthly or quarterly tables... but that's only with the benefit of hindsight. At the time when I was setting up the project, the decision to go with one big table was a conscious one - the worry was that it would make queries across larger date ranges more complicated. Knowing what I know today, I would have gone the other way. "
Revert "Filter out pull requests from issue stats (fixes #10)"

This reverts commit 4c8c5d9a33755535090081cb9873936f662b20b5.
Its not obvious enough how this split works,
since an issue can be closed by a pull request,
and they show up the same through the API as
a pull request which created an issue in the first place.
Updated octokit, added faraday http caching