added methods for vote link classes

<h3>Assignment for *Another Interlude*</h3>

>Define a vote_link_classes method which takes two arguments: a post and a direction string -- either 'down' or 'up' -- to determine which chevron icon to display. The method should return a string with all the appropriate classes for that link.

>Break this behavior into two helper methods: up_vote_link_classes and down_vote_link_classes. Each will only need a post as an argument, and will be less logically complex than a consolidated vote_link_classes would be.