Remove setters from UserInterface

Setter methods are potentially unnecessary and may vary per implementation due to PHP's limitations on type-hinting and method signatures.  For instance, if I have an `EmailAddress` value object, I'll want to construct my user entity like so:

```php
// ...
public function setEmail(EmailAddress $email)
{
    $this->email = (string) $email;
}
// ...
```
A fatal error occurs:
```
Fatal error: Declaration of My\Entity\UserTrait::setEmail(EmailAddress $email) must be compatible with ZfcUser\Entity\UserInterface::setEmail($email)
```
Also, if visibility is changed on the setters, fatal errors occur.  In DDD entities, the `$id` is immutable:
```
Fatal error: Access level to My\Entity\UserTrait::setId() must be public (as in class ZfcUser\Entity\User)
```