Add Sheets, Gmail & Use NIO as a backend?

[adiwajshing]

Hi,

Since the goal of this library is also to be cross-platform and for server side applications, wouldn't it be a good idea to shift to NIO as the base for all requests instead of using completion handlers?

Also, are ya'll planning on adding Gmail & Sheets support?

I required all these objectives to be fulfilled for a Swift backend, but since there wasn't such a library, I built one myself. I could merge this here, if that's something you're looking to incorporate?