Total rewrite - using Module#prepend

Hacking protected method #read_entry via Module#prepend and
Class#inherited to ensure all subclasses first call
BattleShip#read_entry, which increments. Makes code _much_ simpler.
Biggest problem that results is having to guess at namespace when get a
miss. Hoping users either pass options[:namespace], or name their keys
with ClassName_anything, so will take classname (ie, whatever comes
before first underscore).
update README to reflect rewrite