Application.getDefault...Store() to yield Directory instances

[the-overengineer]

Fixed Application.getDefaultAccountStore() and Application.getDefaultGroupStore() to return store (Directory) instances instead of links. Implemented via mixin. Added mixin to Organization, as well. Fixed tests where applicable, removed some from Organization where it gets too complex to test successfully.

Breaking change, hence merge to 1.0.0.

var application;

application.getDefaultAccountStore(function (err, directory) {
  if (err) {
    throw err;
  }

  console.log(directory.name);
});

Fixes #376

[coveralls]

Coverage decreased (-0.3%) to 89.205% when pulling a9648ac on feature/get-store-return-store into c09a89c on 1.0.0.