Makefile enhanced to error out with instructions if invoked without a target.

`Makefile` enhanced to install required test utility `urchin` on demand locally, via `npm install`. The shell command that locates the urchin executable was amended to also find a locally installed version, not just a global one as before.

Note that invoking just `make` (without a goal) currently (still) only runs the `sh` test (at least on my OS X 10.9.5 machine). Is that the intent?
New target that simply lists all other targets; i.e., `make list` shows all other (non-special, non-hidden) targets defined in the makefile. For an explanation, see http://stackoverflow.com/a/26339924/45375