Create votes for episodes

Currently working for episodes.  Functionality could easily be added for reviews and/or comments. <episode>.vote_score returns voter score (with upvotes counting as 1 and downvotes counting as -1).
> If you want to be sure that an association is present, you'll need to test whether the associated object itself is present, and not the foreign key used to map the association.

http://guides.rubyonrails.org/active_record_validations.html#presence

```ruby
# bad
validates :user_id, presence: true

# good
validates :user, presence: true
```