User can create categories

User can create a category
User can view a category
User can edit a category they created

Added categories controller, category model, category create form partial, edit view, edit category form partial, new view, show view. 

Added error partial.
this seems a little insecure--if the user can stop the redirect they can still submit the form. why not do
'''@category = current_user.categories.find(params[:id])''' 
so it will return a 404 if they don't own the category? we can alter moderators to make this work later