WIP: add fixtures to not bother api.github.com when testing

This adds cached copies of HTTP responses from https://api.github.com (similar to other [octokit libraries](https://github.com/octokit/octokit.rb)) so CI testing does not need to actually talk to GitHub. These will be periodically updated as changes to GitHub's API occur.

Currently these work for the NodeJS tests (which Travis-CI runs) using [linkedin/sepia](https://github.com/linkedin/sepia) which is similar to [vcr/vcr](https://github.com/vcr/vcr) but there is/will-be support for running the tests in the browser.