Add Votes controller specs

Create tests for each Votes controller action. Expect each to
appropriately modify the vote resource then render the 'idea/show'
template back to the user.
Fill out Votes controller actions

Fill in the 'create,' 'update,' and 'destroy' actions to modify vote
resources belonging to a particular idea and user. Each action calls a
method on the Idea to be modified in order to increment or decrement
the Idea's rating.