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.

Also: **PLEASE PLEASE PLEASE** give this a quick once-over on iOS 7 and 6. I don't have devices running either OS immediately available to me. I've been very defensive in my coding, and it shouldn't cause any regressions, but I wanted to flag this as a potential concern.
This method is available only in iOS8 SDK, so I'm guessing the PR requires Xcode 6 to be built against?  Are we cool with that?    I know in practice switching to iOS8 SDK on several project caused a number of visible and invisible issues.

To get this compile using Xcode 5, we can use some objc runtime features to dynamically detect if this method exists and call it then, at runtime.