Version for older versions of android

[arthurlutz]

As discused in #3, I'd be interrested in getting this application working for much older versions of android.

I think this has a nice potential for reuse of older hardware that can't be updated.

I have a bunch of old phones/tablets that I can help testing. For example one of the oldest devices is a 2.3.7 version of Android. (but also 4.1.1 and 6.0).

I know this is a technical challenge and I'd be happy to have way less features on older versions of the application.