Adding examples for HTTP/2 framing.

Motivation:

Provide some example code to show how to bootstrap client and server for
use with HTTP/2 framing.

Modifications:

- Fixed Http2ConnectionHandler to allow headers after stream creation.
Needed for response headers.

- Added toString() to all frame classes to help with debugging/logging

- Added example classes for HTTP/2

Result:

HTTP/2 connections now properly support response headers. Examples for
HTTP/2 provided with the distribution of examples module.

After your change, what will change.