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).
You can turn this into a named route by adding the `as` option on the end.

```ruby
put "episodes/:id/vote/:vote_value", to: "episodes#vote", as: :episode_vote
```

You can also nest this under the other episodes routes to make it easier to see that they're related:

```ruby
resources :episodes do
  member do
    put '/vote/:vote_value', to: 'episodes#vote', as: :vote
  end
end
```

In either case, assigning the route a name makes it easer to use in your views:

```html
<%= link_to "upvote", episode_vote_path(@episode, 1), method: :put %>
<%= link_to "downvote", episode_vote_path(@episode, -1), method: :put %>
```