Return promise in modelActions.validate

[naomiajacobs]

So that it matches the signature of the others?

[juanca]

Yeah, and we can potentially drop preValidateAction and postValidateAction since async actions should really be promises which can be chained (on usages): flexibility! modularity!

E.g.

 modelActions.validate('some_collection', attributes, {
    preValidateAction: someActionBeforeValidation,
    postValidateAction: someComplexActionThatActuallyDoesManyThings
});

vs

dispatch(someActionBeforeValidation);
modelActions.validate('some_collection', attributes)
  .done(someSuccessAction)
  .fail(someFailureAction)
  .always(someCleanupAction);

[juanca]

And with promises, done callbacks can be broken up since it can queue up many functions!

[juanca]

This is related to #4