Make it possible to close standard ad formats

[ericholscher]

I think it would be good to be able to close the ads. Some of our publishers have implemented this themselves, but we should at least make having it be "closable" an option, if not the default for all our client implementations.

One design decision is whether to also have this not display ads to this user for some period of time. Perhaps set a 1hr cookie on the user with this preference on the site they are browsing? This goes against our "no cookies" marketing, but I think this seems like a pretty benign one.

[ericholscher]

@davidfischer thoughts on this one? If it isn't a lot of work, I think this is probably worth doing soonish, just to show good will in the community.

[davidfischer]

I think closing is fine. I don't think we should set any cookies at all -- even 1 hour ones. I think it will just yield complicated messaging.

[davidfischer]

#94 has an example of a close-able ad.