模型过滤器
一个插件，过滤器从属性模型。
基本用法
var modella = require ( 'modella' ) ,
filter = require ( 'modella-filter' ) ;
var User = modella ( 'User' ) ;
User . use ( filter ) ;
User . attr ( 'username' )
. attr ( 'email' )
. attr ( 'password' )
var user = new User ;
user . set ( { username : 'JimBo' ,
email : 'jimbo@bob.com' ,
password : '123456' } ) ;
user . filter ( 'password
