directory on the device.

This work is to support a workflow where developers testing on an iOS
device will automatically see their app updates as they change their Sky
code. Currently this works by using the ios-deploy tool:
$ ios-deploy --bundle_id 'org.domokit.sky.game' --upload out/ios_Debug/game_app.app/app.skyx --to Documents/app.skyx
Upcoming commits will incorporate this into skytool.