support model.set('user.name', 'Joe')

model.set('user', {name: 'George', email: 'george@example.com'})
model.set('user.name', 'Joe'); 
model.get('user')  // =>  {name:'Joe', email: 'george@example.com'}
model.set('user.email', 'Joe@abc.net')
model.get('user')  // =>  {name:'Joe', email: 'Joe@abc.net'}
model.get('user.name') //=> 'Joe'

support access an object attribute with "."