The profiler and benchmarking showed that calling evbuffer_add()
repeatedly is very costly. To avoid this, we buffer up small writes
into one large chunk and call evbuffer_add() less times.

some small additions for clarity. please check to ensure meaning is preserved.

automake version is a bit picky about installation location and uses
different compiler flags from distutils. We prefer official distutils.
Now nghttp2.c, which is generated by cython, is distributed in
archive.

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

The statistics of pushed resources are also calculated.

This is very useful to associate application specific data to promised
stream.

nghttp2_nv_array_copy now does not complain the header field is large.