Create Messages

you could also validate that receiver and sender aren't the same using http://guides.rubyonrails.org/active_record_validations.html#using-a-proc-with-if-and-unless
```ruby
validates :receiver,
  if: Proc.new { |message| message.receiver != message.sender }
```

I think that code works. I'd check with Gabe / Josh to make sure that that's not a bad practice