improving ad_channels

Hello, Jeremy.

I'm using Ad module to display a variety of ads across my site's pages. Initially, I found idea with Ad groups great, but later I also found some lack of flexibility in controlling where Ad should be shown.

I created a module which allows to control Ad placement just like Drupal's block visibility settings with some improvements. You have three textareas, where you can:
- Show on which pages ad SHOULD be displayed.
- Exceptions to previos rule.
- PHP visibility rules (which can replace or be added to first two).

This set of rules provide complete control on Ad placement, so I think it should be included into main package or even merged to ad.module itself.