Doing inflation after error produces invalid results, especially, if
it is in NGHTTP2_HD_STATE_READ_INDEX, the inflater->left could be 0,
which causes assertion error.  Add sanity assertion for index

Also erase generated .rst files on `make clean`.

Patch from Dave Beckett

The only drawback is that page source only contains include
directive. We disabled page source for now.

Which was moved to spinx_rtd_theme/static/fonts

Patch from Dave Beckett

These are promoted to public API from src/http2.h

Only stream ID which larger than currently used stream ID is
detected as idle.

It is easy enough to check the end of incoming data by evaluating
frame->hd.flags & NGHTTP2_FLAG_END_STREAM in on_frame_recv_callback

This makes the callback semantics consistent with the other
frames like HEADERS/PUSH_PROMISE.

Previously, there is inconsistency when on_frame_recv_callback
is called between HEADERS/PUSH_PROMISE and the other frames.
For former case, it is called before header block, in latter
case, it is called after whole frame is received. To make it
consistent, we call on_frame_recv_callback for HEADERS/PUSH_PROMISE
after its frame is fully received. Since on_frame_recv_callback
can signal the end of header block, we replaced on_end_headers_callback
with on_begin_headers_callback, which is called when the reception
of the header block is started.