min validator

var SomeModel = Backbone.Model.extend({
  validation: {
    age: {
	  min: 1
	}
  }
});

• non numeric value is invalid
• number lower than min is invalid
• number equal to min is valid
• number greater than min is valid
• numeric string values are treated as numbers
• NaN is valid
• when required is not specified null is invalid
• when required is not specified undefined is invalid
• when required:false null is valid
• when required:false undefined is valid
• when required:true null is invalid
• when required:true undefined is invalid