This update does several significant things:

1. It eliminates the base Nghttp2* classes and folds those
   in to node::http2::Http2Session and node::http2::Http2Stream
2. It makes node::http2::Http2Stream a StreamBase instance and
   sends that out to JS-land to act as the [kHandle] for the
   JavaScript Http2Stream class.
3. It shifts some of the callbacks from C++ off of the JavaScript
   Http2Session class to the Http2Stream class.
4. It refactors the data provider structure for FD and Stream
   based sending to help encapsulate those functions easier
5. It streamlines some of the functions at the C++ layer to
   eliminate now unnecessary redirections
6. It cleans up node_http2.cc for better readability and
   maintainability
7. It refactors some of the debug output
8. Because Http2Stream instances are now StreamBases, they are
   now also trackable using async-hooks
9. The Stream::OnRead algorithm has been simplified with a
   couple bugs fixed.
10. I've eliminated node_http2_core.h and node_http2_core-inl.h
11. Detect invalid handshake a report protocol error to session
12. Refactor out of memory error, improve other errors
13. Add Http2Session.prototype.ping