Add logic to selectively exclude zones from being passed to OpenX based on block visibility logic

Issue Summary

Use case:
Said site has upwards of 60 zones that are used all over the site, on various pages. On any given page, at most 11 zones load. If we use the current logic, all 60 zones will be passed to OpenX, and if there are potential matches for all of those zones, this adds significant wasted bandwidth.

I suggest we create a hook_openx_filter_zones to run before zones are defined in the JS array that is referenced by OpenX. This way, other modules can refine the list of zones. Would be useful to develop an openx_context module to implement this hook when ad zones are placed via context.

Anyway, at the very least, would make sense to create this hook so other modules can add logic to this process.

Will toss up a patch tomorrow for review/discussion.