Migrate Handler to new plugin style

Note: This is not stable API yet.

Before:

    # lib/droonga/plugin/handler/my_handler.rb

    require "droonga/handler_plugin"

    module Droonga
      class MyHandler < Droonga::HandlerPlugin
        repository.register("my-handler", self)

        command :my_command
        def my_command(message, messenger)
        end
      end
    end

After:

    # lib/droonga/plugins/my_plugin.rb

    require "droonga/plugin"

    module Droonga
      module Plugins
        module MyPlugin
          Plugin.registry.register("my-plugin", self)

          class Handler < Droonga::Handler
            message.type = "my-command"

            def handle(message, messenger)
            end
          end
        end
      end
    end