Better support for inheritance

[mvz]

Right now, inheriting models from a superclass is awkward, because you're supposed to include HapppyMapper in each subclass to copy and inherit the attribute lists defined on the class

So you end up with this:

class Super
  include HappyMapper
end

# And for each subclass:
class SubClass < Super
  include HappyMapper
end

This will work but is awkward:

class Super
  include HappyMapper

  def self.inherited(base)
    super
    base.include(HappyMapper)
  end
end

HappyMapper should really define the inherited hook itself.