default lambdas should pass in the object

[mustmodify]

class Person < Valuable
  has_value :first_name
  has_value :last_name
  has_value :initials, :default => {|person| person.first_name[0] + person.last_name[0] } # needs some nil checking
end

[mustmodify]

@andrewcsmith wants :lazy => true to run on demand rather than on init