Improve Makefile

First of all, hi. I've been thinking for a while about a window manager with a small C core and embedded Guile for scripting purposes. I was really happy to see that someone else has had the same idea and has actually gotten it up and running. So, rather than re-inventing the wheel and giving the world yet another window manager, I'd like to help out with yours. I think the concept holds great possibilities compared to other WMs out there. So, I hope to contribute here and there when I have the time. In the meantime, I'll run nwm and see how it goes.

As a first task, in this pull request I'm submitting a major revision of the Makefile to make it more standard and flexible. I also added the targets "all", "install" and "uninstall". Honestly, I'll probably one day replace it with Autoconf/Automake files, unless you object to that. I think it would be more robust in the long term. 

As an aside, I'm a bit concerned about the name "nwm" since there are other projects with the same name. In particular, there's a Node.js-based window manager here on Github, a personal dwm fork, and there's an ncurses-based window manager selector, all with the same name and residing in the same general area of software. Ultimately, this is a minor quibble and who am I to ask you to change your software's name, but it may be something to think about.