Enhanced Companion: Can now scan and load APK files.

When you package an app and generate a QR Code, you can now use the
Companion to scan that code and it will call the package
installer. This means that you no longer need an external bar code
scanner to install packaged files from App Inventor. The Companion
provides you all that you need!

Also fixed #520. If you start the Companion when you don’t have a WiFi
connection, you will receive an Error Dialog box after which the
Companion will exit. No WiFi connection is needed for a USB
connection.

Change-Id: Ia0d96b1a7e49fd6e3fc5801ad86c9124adbe0a71
Merge branch 'master' of https://github.com/mit-cml/appinventor-sources