Add notice if user has already voted in that category.

# Changes
 - Add notice to category page if user has already voted in that category. Closes #21.

# Screenshot
![screen shot 2014-11-04 at 1 00 57 pm](https://cloud.githubusercontent.com/assets/583202/4905073/a1f3fef2-644c-11e4-8945-b9080a68c305.png)