Properly request the user's location on iOS 8. 

Specifically requests the "when app is in use" permission, as opposed to the always permission. I don't think we need the location to always update, regardless of whether the user has the app open. Fixes #326

One thing I've noticed from surreptitiously watching other people use the app is that they routinely kill it when they're done using it. My guess is because the app keeps using GPS for several seconds after it's backgrounded. The users don't need to do this, and hopefully this will wean them off the behavior.