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).
We should probably also make sure that a user can only vote on one particular review/episode once. You can use the uniqueness validation (coupled with a compound index):

```ruby
validates :voteable, uniqueness: { scope: :user }
```

http://guides.rubyonrails.org/active_record_validations.html#uniqueness