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).
Since we want to make sure that each user can only vote on each voteable once, we can create a unique index to make sure that it's impossible to submit duplicates.

Multicolumn indexes also will help speed up the queries that will be done often here, like finding a vote from a specific user for a specific voteable.

http://www.postgresql.org/docs/8.2/static/indexes-multicolumn.html

```ruby
add_index, :votes, [:voteable_id, :user_id], unique: true
```

For some awesome insight into indexes, check out: 

- https://tomafro.net/2009/08/using-indexes-in-rails-index-your-associations
- https://tomafro.net/2009/08/using-indexes-in-rails-choosing-additional-indexes