Supporting github enterprise

Of all the github api libs, I find this one to be the most well written. I've been using it for working with gihub enterprise and it works fine for most gets.

However, it fails on POST/PUT where:
- SSL errors crop up due to enterprise being signed by an internal certificate signing authority
- no ability to pass extra headers

Above 2 can be solved if there was a way to pass on additional headers for the request. Currently I'm using request-promise and passing the CA chain as well as custom headers when POST/PUT are needed but this is sub optimal solution.

I'm using this in a Node (v4.4.3 and above) app.

No files or tests are named. Start by tracing how POST and PUT requests are constructed, then review the existing request-promise usage and how request options could carry a CA chain and additional headers; done means Enterprise POST/PUT requests can use both without the reported SSL failures.

索引モデルが issue の本文から書いたものです。