Implemented dependent attribute (double handlebars)

Supports both dependent attribute defined as a string or as lazy
attribute.
Dependent attribute may also contain associations.

Example:

Factory.define('job', Job, {
      title: 'Engineer'
    , company: function(cb) { cb('Foobar Inc.'); }
    , description: function(cb) { cb('{{title}} in {{company}}'); } //
--> "Engineer in Foobar Inc."
    , description_alternate: '{{title}}, {{company}}' // --> "Engineer,
Foobar Inc."
    });