Thread-safe class inheritable attributes

Allows for defining class-level attributes whose values are:

• inherited by subclasses,
• stored in a thread safe manner,
• accessible via calls to super,
• stored in a memory efficient manner (the registry shrinks itself on nil values).

Example usage

require 'class_inheritable_attributes'

class Resource
  class_inheritable_attr_accessor :site, :timeout

  def self.timeout
    super || 10
  end

  def self.timeout=(seconds)
    super(seconds.to_i)
  end
end

Resource.timeout  # => 10
Resource.timeout = "5"
Resource.timeout  # => 5

class AccountingResource < Resource
  self.site = "http://account.example.com"
end

class Balance < AccountingResource
end

# Child's value defaults to parent's
AccountingResource.site   # => "http://account.example.com"
Balance.site              # => "http://account.example.com"

# Child can set its own value
Balance.site = "http://balance.account.example.com"
AccountingResource.site   # => "http://account.example.com"
Balance.site              # => "http://balance.account.example.com"