Managing ad flight targeting

[davidfischer]

Advertisers should be able to manage the targeting of their flights. At a basic level, they should be able to add/remove/update countries, programming languages, and possibly keywords to their targeting.

Where this becomes tricky is that changing some of the targeting could result in the prices for things changing. For example, if an ad flight targeted North and South America and changes to just North America, the price per click and price per view change.

One possible solution (and one that I favor) is to change slightly how our ad accounting works where each click and view just debits the funds left on a campaign/flight. This would make adding new countries to the targeting simple and easier.

Depends on #87