recording YAML parse and emit events.
* ext/psych/lib/psych/handler.rb: adding a list of events so that
handler classes can more easily be meta-programmed.
* test/psych/handlers/test_recorder.rb: tests for the change.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37378 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
