Make it easier to include modules into backend class

[shioyama]

This is a simple change which allows plugins to simply define modules BackendMethods and/or BackendClassMethods and have the backend class auto-include/extend these modules. This simplifies something like this:

module MyPlugin
  extend Mobility::Plugin

  included_hook do |klass, backend_class|
    backend_class.include BackendMethods
    backend_class.extend BackendClassMethods
  end

  module BackendMethods
    # ...
  end

  module BackendClassMethods
    # ...
  end
end

To just this:

module MyPlugin
  extend Mobility::Plugin

  module BackendMethods
    # ...
  end

  module BackendClassMethods
    # ...
  end
end