Add the ability to install the app into SD

[GrazianoCapelli]

Some users asked to add the possibility to install/move the app into SD instead of the internal memory of the device, depending on user choice.

The installation into real SD should save a small amount of device space (less than 2 MB), because all the app data must still be saved into the device memory (this is an Android restriction). Moreover, with the app on SD, when the user enables USB mass storage to share files with a computer or unmounts the SD card via the system settings, the external storage is unmounted from the device and all applications running on the external storage are immediately killed (Thus the app will stops working, and the service will be killed).

Here you can find the Google documentation about the App Install Location.

I'm a little skeptic about the implementation of this feature. For now I open the issue to take track of this need and, of course, to collect other opinions.