Validate :email in User model using email_validator gem

#747: I planned to validate `:email` using some regex expression, but as you can see [here](http://www.aidanf.net/posts/validating-emails-in-rails), it's not so easy - so I used [email_validator](https://github.com/balexand/email_validator) gem - Is it OK? :smile: :email: :christmas_tree: