Unify InputAdapter and OutputAdapter to Adapter

Location is changed.

Before:

    lib/plugin/input_adapter/NAME.rb
    lib/plugin/output_adapter/NAME.rb

After:

    lib/plugin/NAME.rb # <- This file has adapter

Registration API is changed.

Before:

    class MyInputAdapter < Droonga::InputAdapter
      repository.register("my-plugin", self)

      command :my_command
      def my_command(input_message)
      end
    end

    class MyOutputAdapter < Droonga::OutputAdapter
      repository.register("my-plugin", self)

      command :my_command,
              :pattern => ["replyTo.type", :equal, "my_command.result"]
      def my_command(output_message)
      end
    end

After:

    module MyPlugin
      class Adapter < Droonga::Adapter
        plugin.name = "my-plugin"
        message.input_pattern = ["type", :equal, "select"]

        def adapt_input(input_message)
        end

        def adapt_output(output_message)
        end
      end
    end

catalog.json is changed.

Before:

    {
      ...,
      "input_adapter": {
        "plugins": ["my-plugin"]
      },
      "output_adapter": {
        "plugins": ["my-plugin"]
      }
    }

After:

    {
      ...
      "datasets": {
        "DATASET": {
          ...,
          "plugins": ["my-plugin"]
        }
      }
    }