add ruby 2.0 to travis
Add deferred signal-handling (fixes #332).

This uses a thread-local queue and a self-pipe so the code in the
trap blocks is properly re-entrant.

For details, see:

  * http://cr.yp.to/docs/selfpipe.html
  * http://blog.rubybestpractices.com/posts/ewong/016-Implementing-Signal-Handlers.html