Base: ReturnValue load/get json string
Validator: Add constraint Url
Validator: Allow constraint Url treat relative url without 'HTTP'
Validator: Make method getConstraint() public
Html: Migrate class/validator.php

To Fwlib\Html\FormValidator

Also created js/form-validator.js, most js operate moved in it, only a few
init script is generated through FormValidator.

A few common used css moved to css/default.css, other js and css are
abandoned, can be inherit/extend in product project.
Base: Better default value of message param in constructor

If message param is obmitted when doing 'new ReturnValue', it will use default
value null . But if the instance is encoded to json then load with loadJson
method, 'message = null' will cause exception because a null value will return
false in 'isset' check, so change the default value of message param in
constructor to empty string ''.