publish prebuilt binaries via github releases

[yocontra]

Currently we publish to S3 - I'm footing the bill for this on my personal account. Wasn't an issue before since I just made this fork for my own usage, but now that it has started gaining more and more downloads the bandwidth has exceeded the free tier. Publishing to github will be more reliable anyways, since it detaches this project from my personal amazon usage and will allow it to be forked freely without having to handroll the prebuild publishing again.

[weagle08]

Hey @contra,
I'm working on the rewrapping of GDAL using N-API (finally) and have removed all the node-pre-gyp stuff for now since it is just another library to learn that is not needed while I work the re-wrap so if you can hold off on a solution to this until I get it complete (hopefully in the next couple of months) you could then apply whatever change you propose to it then so that it won't have to be reapplied?

[yocontra]

@weagle08 Sure, that seems fine!