Don't require plugin name for each adapter

Before:

    require "droonga/adapter"

    module Droonga
      module Plugins
        module MyPlugin
          class Adapter < Droonga::Adapter
            plugin.name = "my-plugin"
          end
        end
      end
    end

After:

    require "droonga/plugin"   # <- Add
    require "droonga/adapter"

    module Droonga
      module Plugins
        module MyPlugin
          Plugin.registry.register("my-plugin", self) # <- Add
          class Adapter < Droonga::Adapter
            # no plugin.name = "my-plugin"
          end
        end
      end
    end