Upvoted functionality. Bringing #49 up to speed.

I've tried to bring pull request #49 up to speed against latest master.

First time working with the lobsters source code so would appreciate if someone took a look at it.

The tests are failing locally for me, but the same fails exists in latest master. I think it is because I used an sqlite database to get the development environment up fast. The errors are sqlite exceptions.

```
Failures:

  1) StoryRepository#top filtering 2 weeks should not include #<Story id: 1, created_at: "2014-07-04 11:24:24", user_id: 1, url: "http://example.com/0038", title: "story title 0038", description: nil, short_id: "dnkczb", is_expired: 0, upvotes: 1, downvotes: 0, is_moderated: 0, hotness: #<BigDecimal:7fd57231b268,'0.0',9(36)>, markeddown_description: nil, story_cache: nil, comments_count: 0, merged_story_id: nil>
     Failure/Error: it { should_not include month_ago }
     ActiveRecord::StatementInvalid:
       SQLite3::SQLException: near "2": syntax error: SELECT "stories".* FROM "stories"  WHERE "stories"."merged_story_id" IS NULL AND "stories"."is_expired" = 0 AND (created_at >= (NOW() - INTERVAL 2 WEEK))  ORDER BY (CAST(upvotes AS integer) - CAST(downvotes AS integer)) DESC
     # ./spec/models/story_repository_spec.rb:156:in `block (5 levels) in <top (required)>'

  2) StoryRepository#top filtering 2 month should include #<Story id: 1, created_at: "2014-07-04 11:24:24", user_id: 1, url: "http://example.com/0040", title: "story title 0040", description: nil, short_id: "uzylea", is_expired: 0, upvotes: 1, downvotes: 0, is_moderated: 0, hotness: #<BigDecimal:7fd572500ad8,'0.0',9(36)>, markeddown_description: nil, story_cache: nil, comments_count: 0, merged_story_id: nil>
     Failure/Error: it { should include month_ago }
     ActiveRecord::StatementInvalid:
       SQLite3::SQLException: near "2": syntax error: SELECT "stories".* FROM "stories"  WHERE "stories"."merged_story_id" IS NULL AND "stories"."is_expired" = 0 AND (created_at >= (NOW() - INTERVAL 2 MONTH))  ORDER BY (CAST(upvotes AS integer) - CAST(downvotes AS integer)) DESC
     # ./spec/models/story_repository_spec.rb:162:in `block (5 levels) in <top (required)>'

  3) StoryRepository#top filtering 2 month should not include #<Story id: 2, created_at: "2013-10-04 11:24:24", user_id: 2, url: "http://example.com/0043", title: "story title 0043", description: nil, short_id: "qqnw8x", is_expired: 0, upvotes: 1, downvotes: 0, is_moderated: 0, hotness: #<BigDecimal:7fd572702958,'0.0',9(36)>, markeddown_description: nil, story_cache: nil, comments_count: 0, merged_story_id: nil>
     Failure/Error: it { should_not include ten_month_ago }
     ActiveRecord::StatementInvalid:
       SQLite3::SQLException: near "2": syntax error: SELECT "stories".* FROM "stories"  WHERE "stories"."merged_story_id" IS NULL AND "stories"."is_expired" = 0 AND (created_at >= (NOW() - INTERVAL 2 MONTH))  ORDER BY (CAST(upvotes AS integer) - CAST(downvotes AS integer)) DESC
     # ./spec/models/story_repository_spec.rb:163:in `block (5 levels) in <top (required)>'

  4) StoryRepository#top ordering orders by votes difference
     Failure/Error: expect(scope).to eq [s3, s2, s4, s1]
     ActiveRecord::StatementInvalid:
       SQLite3::SQLException: near "12": syntax error: SELECT "stories".* FROM "stories"  WHERE "stories"."merged_story_id" IS NULL AND "stories"."is_expired" = 0 AND (created_at >= (NOW() - INTERVAL 12 MONTH))  ORDER BY (CAST(upvotes AS integer) - CAST(downvotes AS integer)) DESC
     # ./spec/models/story_repository_spec.rb:176:in `block (4 levels) in <top (required)>'
```

Hopefully this can be merged this time.
I think the `has_many :voted_stories` should be specifying these conditions.  That would get rid of the (ambiguous) upvoted method and gain from Rails' caching of the association.