Improve Makefile and add install target

Now, users can run 'sudo make install' to install NodeAtlas and all
resources to `/usr/local` (by default). In order to install to
elsewhere, invoke `prefix=/usr sudo make install` (for example).

Note that in the installed version, NodeAtlas will look for its
configuration at /etc/nodeatlas.conf, but that file is not created for
you. (See #169.)

If you want to use old values of `res/` and `conf.json`, you can just
build with `go build`.