Add alternate constructor support to aggregations

The composed_of macro assumes that the value object can be instantiated with new. But for some aggregate classes, new is not the appropriate constructor and instead something like parse is required.

This patch adds support for a :constructor option to the composed_of method. It accepts a symbol identifying a constructor method on the aggregate class. All aggregate object are then instantiated using this constructor method.

A typical example of this new functionality:

require 'uuid'

class Widget < ActiveRecord::Base
  composed_of :uuid, :constructor => :parse
end

Of course, the default value for the constructor is :new.

[This is identical in function to the enhancement documented on Trac ticket 11122. It has been updated to work on Edge and includes less brittle tests and a git-compatible patch.]