fixes #4, fixes #5: Fetch email addresses for contributors

* Attempt to get the email address for each contributor by looking at
  the most recent commit from that contributor in a repo

* GITHUB_CLIENT_ID and GITHUB_CLIENT_SECRET env vars to set OAuth
  details, which gets us a higher rate limit from the API

* Caching GET wrapper for github repo API calls
Merge pull request #7 from lmorchard/github-keys-5

fixes #4, fixes #5: Fetch email addresses for contributors