Change protocol ID to h2-13
Use hex in nghttp2_error_code and nghttp2_settings_id
Allow multiple transmission of GOAWAY frame
Rename last_stream_id in nghttp2_session to remote_last_stream_id

This commits also fixes last stream ID in GOAWAY uses wrong stream ID.
Don't send GOAWAY with last stream ID larger than the value previously sent
Support graceful shutdown using multiple GOAWAY

Add last_stream_id parameter to nghttp2_submit_goaway().  To terminate
connection immediately with application chosen last stream ID,
nghttp2_session_terminate_session2() was added.
Treat unknown error code as NGHTTP2_INTERNAL_ERROR
Add test for graceful shutdown use case
nghttpx: Require TLSv.12 for h2
Update README.rst
doc: Update reference
python: Bump up HPACK draft version to 8
python: Add -c, --simulate-table-size-change option to hpackmake.py
examples: Check return value of evbuffer_drain
examples: Fix libevent-server which could not send error reply
doc: Update tutorial