required validator

var SomeModel = Backbone.Model.extend({
  validation: {
    name: {
	  required: true | false
	}
  }
});

var SomeModel = Backbone.Model.extend({
  validation: {
    name: {
	  required: function() {
		return true | false;
	  }
	}
  }
});

• empty string is invalid
• non-empty string is valid
• string with just spaces is invalid
• null is invalid
• undefined is invalid
• false boolean is valid
• true boolean is valid
• required can be specified as a method returning true or false