Add http2 support

[doug-wade]

Http2 has landed in node.js. We should add support.

[nickserv]

The API level for it is 1 (experimental), we might want to wait until this changes to put it in a stable release. Also, does this replace #16 or should we still fix that first?

[doug-wade]

I don't think we should wait, I think we should add the feature now but make sure its completely optional, so clients can test and develop against the http2 api sooner without having to risk it in prod.

[nickserv]

Good point, though I don't think we should make this block any releases until it's no longer experimental.

[doug-wade]

Afaict, this is a documentation-only fix. Iiuc, since we extend koa, clients should be able to follow these instructions and have http2 support. I'm going to test and document.