contributor api binding

[sevenEng]

try to solving issue #112

according to the github doc https://developer.github.com/v3/repos/statistics/#get-contributors-list-with-additions-deletions-and-commit-counts, request would be responded with 202 (and empty body) when the contributor data need to be calculated and haven't been cached, under this situation, return an empty list

omitted some fields in the response, suggestions welcome : )

[samoht]

I've restarted the failed TravisCI build. Not sure why camomille failed on Windows..

[dsheets]

Camomile is still failing on Windows. I will merge this anyway.

Thanks, Qi Li, this is a nice contribution and really helps. I have a few nitpicks but I will address them in a subsequent PR.

[sevenEng]

Glad I can help : ) Thanks for merging 👍