C0003 list subcategories

Adding this logic a little different from what you would do in a simple rails app. I had to use jquery to create a POST request when the dropdown changed. If you don't have knowledge of jquery and json let me know so that I can explain the code with more detail.

To test this, navigate to: http://localhost:3000/add_listing
You will see the 2 drop downs. Just change the category option and it will automatically refresh values for the subcategories.
@ab22 Is this what I would use for the path that I would use to view all the Listings in this subcategory?

I'm trying to use it like this but it's not working:
```
<%= link_to subcategory.name, subcategories_find_by_category_path(subcategory) %>
```