kgio 2.10.0 - slimming down...

The largely-unused autopush functionality is now a no-op.
Existing applications will probably not notice, and are
better off using MSG_MORE anyways if they're on Linux.

Our poll(2) wrapper now uses rb_hash_clear if available,
reducing global method cache dependence.

shortlog of changes since 2.9.3:

      README: remove reference to Rainbows!
      writev: update comment on preserving errno
      poll: avoid rb_funcall for rb_hash_clear
      minor doc updates
      remove autopush support and make it a no-op
      HACKING: update URL for Message-ID lookup