Banner module was seperated from core to make it easier for module
maker to build replacement modules ..however for now if you do want to
do that, you need to respect 3 things.

1. Your module should also be called banners
2. The function file that displays the banners should be:
   modules/banners/include/functions.php
3. The function to call to show a banner should be: showbanner($option)

All the rest is up to you ;-)