Feature/cleanup user model

/to @ritch 
/to @bajtos 

Two issues to be fixed by this PR:
1. https://github.com/strongloop/loopback-connector-mysql/pull/43
2. The current model is invalid, if we create user with credentials/challenges properties, it will throw.
Coding style: put the closing curly brace at a new line.

```
User.create({
  email: 'f1@b.com', password: 'bar1',
  credentials: {cert: 'xxxxx', key: '111'},
  challenges: {x: 'X', a: 1}
}, function (err, user) {
```