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.
@isjaylee Nop. This is a post call to get all Subcategories that belong to a Category. You can try changing the via :get and navigate to http://localhost/subcategories/find_by_category?category_id=1 or any valid category as parameters. It will then return a json with an array of subcategories.