Add update and destroy to categories

https://trello.com/c/uYliNeqU/22-admin-can-edit-destroy-categories
![selection_006](https://cloud.githubusercontent.com/assets/8248100/4921130/db6b24c4-6507-11e4-820a-e135138b0212.png)
![selection_007](https://cloud.githubusercontent.com/assets/8248100/4921140/fe020142-6507-11e4-8ef5-aee56d6b3b7f.png)

No need for instance variables here since you will redirect away, rather than render a view. Instance variables are used to pass models to the views.