fixed sbcl setup problems

I am working on a Ubuntu 13.04 64 Bit system with SBCL 1.1.10 (compiled from source) and I couldn't setup stumpwm. The main problem was, that it could't find `stumpwm` in the `make-image.lisp` phase.

The important change, to make it build the `stumpwm` file, was to add `--load ./stumpwm.asd` to the `Makefile`.

Thie patch worked on my system... I am not sure if this will help others, but still, here is my pull request.