260.install alethzero on unix

...This is an attempt to fix #260 with almost no understanding of cmake.

I have tested this on my platform as far as getting the gui to pop up by typing only ``alethzero`` which is now installed into ``/usr/loca/bin`` (via GNU `stow` on my system).

However, I urge someone who understands `cmake` and the other platforms to carefully vet this before accepting this pull request, because I am just doing mimicry cut'n'paste here without careful understanding.