Support for cross-shell testing added, via option `-s <shell>` and env. variable `TEST_SHELL`.

* For **tests that _source_ shell scripts**: **option `-s <shell>`** now tells urchin to invoke test scripts with the specified shell (only shebang-less and `#!/bin/sh` tests scripts).
* For **tests that _invoke_ schell scripts**: instruct users to write their tests to always **invoke via environment variable `TEST_SHELL` (e.g., `$TEST_SHELL ../foo`)**, and invoke urchin with that variable defined as needed, e.g., `TEST_SHELL=ksh urchin ./tests`; urchin defaults `TEST_SHELL` to `/bin/sh`.

See updated `readme.md` for details.
Hint added to read-me re standalone invocation of tests that use TEST_SHELL; revision of wording in read-me; argument checking no longer treats a missing dir. argument as an explicitly specified argument.
Fixed typo in read-me.