Board owners

Card: http://catchup.io/boards/3/cards/79

### How to use this feature

Who creates a board has, naturally, access to it. If we need to grant access for multiple users we need to access Rails console for now and do:

```ruby
board.add_owner(User.first)
```

Ideally, in the future, we will have a nice UI to manage these kinds of auth behaviors.

/cc: @exalted