Does there exist an iPhone application that can call an online API (RESTful or otherwise) triggered by geofences? I've pretty much stated the entire question in the title:

I'm looking for an iPhone application that can call an online API (preferably RESTful but I'll accept anything) as a reaction to triggering a geofence.

So, as a stupid example, a tweet could be sent out automatically when I arrive at my house. The iPhone would see the geofence trigger, the application would be invoked, which would call the Twitter API to send the tweet.
Looks like Locative can do this:

https://itunes.apple.com/us/app/locative/id725198453?mt=8

https://github.com/LocativeHQ