Build from source/support 2.8.0

[dzomaya]

Right now, most repos are still serving NUT 2.7.4 instead of the latest version 2.8.0. Since the script uses apt, that means most users will get 2.7.4. Building from source or otherwise getting 2.8.0 would allow users to get the latest stable build of NUT instead of 2.7.4. At least one user has already asked: https://github.com/dzomaya/NUTandRpi/issues/1

We could also make the script more portable (right now, it depends on apt which limits us to the Debain-y stuff that uses apt).

The 2.8.0 problem solves itself when/if the default repos start serving 2.8.0 (I expect that "soon", but not sure how long).