Benchmark shows 10% faster with libev compared to libevent.  Also
response time in high load condition is much faster.

and it is now valided by http://lint.travis-ci.org/

(Hooray, your .travis.yml seems to be solid!)

Initially, we use nghttp2_stream.data_item to refer only item with
DATA frame.  But recently we use it to refer HEADERS frame as well.
So it is better to call just item rather than data_item.  This applies
to all related functions.