add preference to limit image file to 4GB (that's the FAT file size limit)

The "External Storage" is often a FAT filesystem, but it turns out that it
is very difficult to reliably detect what filesystem the External Storage
is using.  So this is a workaround using a manual preference.

fixes #80 https://github.com/guardianproject/lildebi/issues/80
detect if Superuser is installed, if not, prompt user to install it

Lil' Debi currently only works with Superuser, and not SuperSU.  And it
requires an app to provide root access.  This moves the test to the first
screen, since it no longer attempts to get root access, but instead just
looks up the SU apps by packageName.

fixes #112 https://github.com/guardianproject/lildebi/issues/112