Define RWS::Books::Genre class to support Books Genre API
Refactor: User intance var to store genre info instead of class var
Books::Genre.new asks API only at the first time to initialize a givne id
RWS::Books::Genre.root is alias of new with the root genre id '000'
RWS::Books::Genre#children returns Genre objects under the genre
Refactor RWS::Ichiba::Genre#children in readability
RWS::Books::Genre#children fetch children of the specified genre aggressively.
RWS::Books::Genre#search delgates other resource's search according to genre id