Add native event-handler for performance

- specifying extconf.rb as extension in gemspec,
  but it doesn't configure any Ruby C extension, just builds native
  event-handler.
Use ruby handler if native handler not available
Test event handlers
consolidate fprintf()s
Refactor native event-handler
argc < 3 is correct
Add makefile to pass 'gem install'
[travis] test native event-handler
Merge pull request #8 from sorah/native-event-handler

Native event-handler

Fixes #7 "Performance problem"