range validator

var SomeModel = Backbone.Model.extend({
  validation: {
    age: {
	  range: [1, 10]
	}
  }
});

• number lower than first value is invalid
• number equal to first value is valid
• number equal to last value is valid
• number higher than last value is invalid
• number in range is valid
• when required is not specified undefined is invalid
• when required is not specified null is invalid
• when required:false null is valid
• when required:false undefined is valid
• when required:true undefined is invalid
• when required:true null is invalid