Some corrections to chapter 15

* ~~**Chapter 15** [Ajax tutorial](https://developer.mozilla.org/en-US/docs/AJAX/Getting_Started) Probably better than the w3cschool one.~~ 
* ~~**Chapter 15** In 15.1.4 add from django.contrib.auth.decorators import login_required~~
* ~~**Chapter 15** in 15.1.5 typo AND should be *an event handler*.~~ 
* ~~**Chapter 15** id='Cats' typo in 15.2.5~~
* ~~**Chapter 15** There is an indent in on the suggest category view that shouldn't be there. unindent cat_list at the end. Otherwise it'll be a part of ``else``.~~ Also is POST ever really needed?. Is it ever used? Since there isn't any submit button the POST mechanism won't be used, right?
* ~~**Chapter 15** Make sure that you can type in lowercase and still receive words that are not lowercase. in get_category_list it should be `istartswith` instead of `startswith`~~