Add cloud_updater app to iotile-ext-cloud

## Overview

We want a simple app (`IOTileApp` subclass) that can download a script to a device in two different ways:

1. The app should be able to use iotile.cloud apis to check for any deployments that affect the given device and if there is a deployment, download the associated script and program it into the device. There should be an annotated method named `update` that does this all-in-one checking and programming

2. The app should be able to directly pull a script from iotile.cloud by slug and program it into a device. There should be an annotated method named `program_script` that takes a slug pointing to a cloud script, downloads it and programs it to the device.

The actual device programming should be done using the `device_updater`. The way to get a `DeviceUpdater` object is to call `self._hw.app(name='device_updater')`. See #403 for what a DeviceUpdater can do.

There should probably be a few other methods for checking whether any deployments affect a device without programming anything from them.

@dkarchmer This is my proposal for allowing users to download scripts directly from the cloud as well as automatically updating devices based on cloud deployments.