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 Hmm. weird.. I left it so it calls the getSubcategories(getSelectedCategory()); on start. I even left a comment on line 25 lol. I will check on that.