Model refactorings and further expansion of specs

Model refactorings and further expansion of specs

* Changed associations (except for many_to_many)
  to use new shorthand syntax, for sake of clarity.
* Changed "validate" reference to "dm-validations"
  to reflect gem renaming in dm-more.
* Added basic specs for most associations.
* Updated some :class_name options for associations
  (still needs to be gone through and tested more
  thoroughly).
* Added missing class and instance methods in
  Mart::AbstractUpload, Mart::Accounts
* Added zipcode, province aliases in Address class
  (and relevant specs)
* Added provinces alias in Country class (and
  relevant specs).
* Added validations into Country class, made name
  property unique.
* Add spec for State class.
* Pushed down image-related properties/constants,
  out of AbstractUpload and into Mart::Image class.
* Add example workaround for current lack of DM
  many-to-many support in Right class.
* Fixed finder ordering syntax in Tag.
* Re-enabled :save callback in User class.
* Made some Mart::Order specs pending, instead of
  letting them fail.