Vote_link_classes added to application_helper

Refactored the way the classes were being defined in _voter.html.erb. Let me know what you think!
This:

```ruby
current_user.voted(post) && current_user.voted(post).up_vote?
````

can be refactored into:

```ruby
current_user.voted(post).try(:up_vote?)
````

Read about the [try method](http://apidock.com/rails/Object/try) and let me know if you have any questions about it.