Nightly Subsurface AppImage for most Linux distributions

[probono]

> I took a very quick glance at the AppImage, and noticed that if you
> strip the subsurface binary, you can shave off some of the size.

You are right; I assume there are even more ways to optimize for size
(e.g., compile dependencies with only the capabilities that we really
need; check whether all files contained in the AppImage are actually
needed).

> You also seem to have subsurface binary there twice
> (under usr/bin and usr/lib)

That was a mistake, changed the recipe at
https://github.com/probonopd/AppImages/blob/master/recipes/subsurface.sh

> I suppose we should need to multiply that by two, one AppImage for
> 32bit and second for 64 bit systems.

Correct. The hardest part will be to figure out how to do this on a
64-bit build host like the ones I am using on travis-ci. Pull requests
welcome.