Provide proper attribute inheritance when subclassing Couchbase::Model subclasses.

(Pull request to fix this can be found here: https://github.com/couchbase/couchbase-ruby-model/pull/2 )

Currently, Couchbase::Model stores attributes (and views) in a class variable set to a mutable hash. This breaks the expected subclass attribute inheritance behavior that ActiveModel provides.

Example:

class A < Couchbase::Model
  attribute :one
end

class B < A
  attribute :two
end

B.attributes #=> {:one => nil, :two => nil}
A.attributes #=> {:one => nil, :two => nil}
B.attributes.object_id == A.attributes.object_id #=> true

This patch enables proper inheritance when ::Rails is present by using Class#class_attribute and detecting subclass additions to attribute:

class A < Couchbase::Model
  attribute :one
end

class B < A
  attribute :two
end

A.attributes #=> {:one => nil}
B.attributes #=> {:one => nil, :two => nil}
B.attributes.object_id != A.attributes.object_id #=> true