Replace HTTP/2.0 with HTTP/2
Define NGHTTP2_CLIENT_CONNECTION_PREFACE macro

NGHTTP2_CLIENT_CONNECTION_PREFACE has the same content with
NGHTTP2_CLIENT_CONNECTION_HEADER, which is now obsoleted by
NGHTTP2_CLIENT_CONNECTION_PREFACE.
Update tutorial
Define NGHTTP2_CLEARTEXT_PROTO_VERSION_ID

This identifier string is used if HTTP/2 is used over cleartext TCP.