Go's HTTP/2 interop test server

The Go language project is now running an HTTP/2 interop test server at:

   https://http2.golang.org/

It's our hope that this either makes it easier for others to test their
clients, or helps expose our bugs.

This a pure Go implementation, independent from other Google
implementations (usually C++). Code is currently at
github.com/bradfitz/http2.

This will eventually go into Go's standard library, on by default and
transparent to users.

The gophertiles demo is fun: https://http2.golang.org/gophertiles

Any feedback or bugs welcome!

- Brad

(sorry if this shows up as a dup; I hadn't joined the list before sending
the first time)