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).
There's actually a more conventional way to set up "polymorphic" associations when using Active Record 

http://guides.rubyonrails.org/association_basics.html#polymorphic-associations

Conventionally, you'd want to use the columns `voteable_id` and `voteable_type`.

You can then set up the association between votes and voteable items like:

```ruby
class Vote
  belongs_to :voteable, polymorphic: true
end
```

You can also define the inverse of this relationship on the models that can be voted on by taking advantage of the same polymorphic stuff built into Active Record:

```ruby
class Episode
  has_many :votes, as: :voteable
end

class Review
  has_many votes, as: :voteable
end
```

The benefit of following these conventions is that you get some more of those magical Active Record queries. For instance, given any particular episode, you can easily find it's votes:

```ruby
episode = Episode.find(10)

episode.votes # => performs the query for you to find all the votes that have voteable_type 'episode' and 'voteable_id' 10