Add publishing hook that uses github releases instead of s3

[springmeyer]

Just need to figure out how to configure oauth + upload binary assets

• http://developer.github.com/v3/repos/releases/
• curl "https://api.github.com/repos/springmeyer/node-addon-example/releases"

[max-mapper]

https://github.com/rvagg/ghauth ftw

[edgarsilva]

+1,000,000 this would make node-pre-gyp even awesomer!

[springmeyer]

I've thought about this more and I'm no longer planning to support github. The ideal long term would instead be integrated with npm's auth. My feeling is that adding alternative support for github over s3 in the meantime would just complicate the process more.