Huffman decoding is costly. It is faster to do geometric realloc
than calling nghttp2_hd_huff_decode_count to know the length in
advance.

We need jansson >= 2.5 because of s# conversion

See https://github.com/Jxck/hpack-test-case for the json file
format.

See https://github.com/Jxck/hpack-test-case for the json file
format.

Requires unreleased OpenSSL >= 1.0.2

This is because nghttp2 and spdylay now don't allow \r\n in header
value. Require spdylay >= 1.2.3.