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?
This _simply_ expanded variable - evaluated once, right away - receives the full path to the `urchin` executable, while first looking for it among the _local_ npm packages.